diff --git a/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Program.cs b/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Program.cs
index d21cf620..863f0b58 100644
--- a/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Program.cs
+++ b/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Program.cs
@@ -34,6 +34,10 @@ builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
 builder.Services.AddScoped<EnvelopeService>();
 
+// UI Preferences & Theme
+builder.Services.AddScoped<UserPreferencesService>();
+builder.Services.AddScoped<ThemeService>();
+
 // DevExpress WASM
 builder.Services.AddDevExpressWebAssemblyBlazorPdfViewer();
 builder.Services.AddDevExpressWebAssemblyBlazorReportViewer();
diff --git a/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/ThemeService.cs b/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/ThemeService.cs
new file mode 100644
index 00000000..7e32c9d6
--- /dev/null
+++ b/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/ThemeService.cs
@@ -0,0 +1,236 @@
+using Microsoft.JSInterop;
+using Microsoft.Extensions.Logging;
+using EnvelopeGenerator.Application.Common.Dto;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+/// <summary>
+/// Manages the active DevExpress theme and dark mode state for the WASM client.
+///
+/// Initialization order:
+///   1. On first load (before login): InitializeAsync() reads localStorage["signflow.darkMode"].
+///      Falls back to window.matchMedia("prefers-color-scheme: dark") if key is absent.
+///      ThemeName defaults to "blazing-berry" until login completes.
+///   2. After login: LoadFromServerAsync() syncs preferences from the server cache.
+///      - If server has no saved preferences (first login ever), the system dark mode preference
+///        detected locally is written to the server immediately.
+///      - Otherwise server values overwrite local state and localStorage.
+///
+/// Persistence:
+///   - Dark mode toggle → localStorage written immediately + debounced PUT /api/UserPreferences.
+///   - Theme change     → debounced PUT /api/UserPreferences (no localStorage for theme).
+///   - All server writes use the in-memory state — no extra GET before each save.
+///
+/// Error handling:
+///   - InitializeAsync / LoadFromServerAsync errors are logged and rethrown.
+///   - Debounced save errors (ToggleDarkMode, SetTheme) are logged; UI is NOT blocked
+///     because the state change is already applied locally. The error is surfaced via
+///     the SaveError property so the UI layer can show a toast/snackbar if desired.
+///
+/// Consumers subscribe to OnChange and call StateHasChanged (e.g. MainLayout).
+/// </summary>
+public class ThemeService : IAsyncDisposable
+{
+    // ── Allowed themes ────────────────────────────────────────────────────────
+    public static readonly IReadOnlyList<ThemeOption> AvailableThemes =
+    [
+        new("Blazing Berry", "blazing-berry"),
+        new("Fluent",        "fluent"),
+        new("Purple",        "purple"),
+    ];
+
+    private const string LocalStorageDarkModeKey = "signflow.darkMode";
+
+    // ── State ─────────────────────────────────────────────────────────────────
+    private string _themeName = "blazing-berry";
+    private bool _isDarkMode = false;
+    private bool _initialized = false;
+
+    // ── Dependencies ──────────────────────────────────────────────────────────
+    private readonly IJSRuntime _js;
+    private readonly UserPreferencesService _preferencesService;
+    private readonly ILogger<ThemeService> _logger;
+
+    // ── Debounce ──────────────────────────────────────────────────────────────
+    private CancellationTokenSource? _saveCts;
+
+    // ── Change & error notification ───────────────────────────────────────────
+    /// <summary>Fired whenever theme or dark mode changes. Subscribers call StateHasChanged.</summary>
+    public event Action? OnChange;
+
+    /// <summary>
+    /// Set when a background save fails. UI layer can read this to show a toast.
+    /// Cleared automatically on the next successful save or on next toggle/theme change.
+    /// </summary>
+    public Exception? SaveError { get; private set; }
+
+    public ThemeService(IJSRuntime js, UserPreferencesService preferencesService, ILogger<ThemeService> logger)
+    {
+        _js = js;
+        _preferencesService = preferencesService;
+        _logger = logger;
+    }
+
+    // ── Public state ──────────────────────────────────────────────────────────
+
+    public string ThemeName => _themeName;
+    public bool IsDarkMode => _isDarkMode;
+
+    /// <summary>
+    /// The CSS href value for the active DevExpress theme.
+    /// Example: "_content/DevExpress.Blazor.Themes/blazing-berry.dark.bs5.min.css"
+    /// </summary>
+    public string ThemeCssHref =>
+        $"_content/DevExpress.Blazor.Themes/{_themeName}{(_isDarkMode ? ".dark" : "")}.bs5.min.css";
+
+    /// <summary>Bootstrap 5 data-bs-theme attribute value ("dark" or "light").</summary>
+    public string BootstrapTheme => _isDarkMode ? "dark" : "light";
+
+    // ── Initialization ────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Must be called once in OnAfterRenderAsync(firstRender: true) — after JS is available.
+    /// Reads dark mode from localStorage, falling back to system (prefers-color-scheme).
+    /// Safe to call multiple times; subsequent calls are no-ops.
+    /// </summary>
+    public async Task InitializeAsync()
+    {
+        if (_initialized) return;
+        _initialized = true;
+
+        var stored = await _js.InvokeAsync<string?>("localStorage.getItem", LocalStorageDarkModeKey);
+
+        if (stored is not null)
+        {
+            _isDarkMode = stored == "true";
+        }
+        else
+        {
+            // No stored preference — detect system setting
+            _isDarkMode = await _js.InvokeAsync<bool>(
+                "eval",
+                "(function(){ return window.matchMedia('(prefers-color-scheme: dark)').matches; })()");
+        }
+
+        OnChange?.Invoke();
+    }
+
+    /// <summary>
+    /// Called after a successful sender login.
+    /// Syncs theme + dark mode from the server.
+    /// If no preferences exist yet on the server (first ever login),
+    /// the locally-detected system preference is saved to the server.
+    /// </summary>
+    public async Task LoadFromServerAsync(CancellationToken ct = default)
+    {
+        var prefs = await _preferencesService.GetAsync(ct);
+        var isFirstLogin = prefs.GridLayouts.Count == 0
+            && prefs.ThemeName == "blazing-berry"
+            && !prefs.IsDarkMode;
+
+        if (isFirstLogin && _isDarkMode)
+        {
+            // First login: propagate local system preference to server
+            prefs.IsDarkMode = _isDarkMode;
+            prefs = await _preferencesService.SaveAsync(prefs, ct);
+        }
+
+        ApplyPreferences(prefs);
+
+        await _js.InvokeVoidAsync("localStorage.setItem",
+            LocalStorageDarkModeKey, _isDarkMode.ToString().ToLower());
+
+        OnChange?.Invoke();
+    }
+
+    // ── Mutations ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Toggles dark mode, updates localStorage immediately, and schedules a server save.
+    /// The state change is applied regardless of whether the server save succeeds.
+    /// Check SaveError after OnChange fires to detect background save failures.
+    /// </summary>
+    public async Task ToggleDarkModeAsync()
+    {
+        _isDarkMode = !_isDarkMode;
+        SaveError = null;
+
+        await _js.InvokeVoidAsync("localStorage.setItem",
+            LocalStorageDarkModeKey, _isDarkMode.ToString().ToLower());
+
+        OnChange?.Invoke();
+        await ScheduleSaveAsync();
+    }
+
+    /// <summary>
+    /// Sets the active theme and schedules a server save.
+    /// The state change is applied regardless of whether the server save succeeds.
+    /// Check SaveError after OnChange fires to detect background save failures.
+    /// </summary>
+    public async Task SetThemeAsync(string themeName)
+    {
+        if (themeName == _themeName) return;
+        if (!AvailableThemes.Any(t => t.Value == themeName))
+            throw new ArgumentException($"Theme '{themeName}' is not allowed.", nameof(themeName));
+
+        _themeName = themeName;
+        SaveError = null;
+        OnChange?.Invoke();
+        await ScheduleSaveAsync();
+    }
+
+    // ── Private helpers ───────────────────────────────────────────────────────
+
+    private void ApplyPreferences(UserPreferencesDto prefs)
+    {
+        _themeName = AvailableThemes.Any(t => t.Value == prefs.ThemeName)
+            ? prefs.ThemeName
+            : "blazing-berry";
+
+        _isDarkMode = prefs.IsDarkMode;
+    }
+
+    /// <summary>
+    /// Debounced server save. Uses current in-memory state — no extra GET.
+    /// Only theme and dark mode are written; GridLayouts are preserved from cache.
+    /// </summary>
+    private async Task ScheduleSaveAsync()
+    {
+        _saveCts?.Cancel();
+        _saveCts = new CancellationTokenSource();
+
+        try
+        {
+            await Task.Delay(500, _saveCts.Token);
+
+            // Use cached preferences to preserve GridLayouts — no network GET needed.
+            var prefs = _preferencesService.GetCached() ?? new UserPreferencesDto();
+            prefs.ThemeName = _themeName;
+            prefs.IsDarkMode = _isDarkMode;
+
+            await _preferencesService.SaveAsync(prefs, _saveCts.Token);
+        }
+        catch (TaskCanceledException)
+        {
+            // A newer save was scheduled — this one is intentionally cancelled.
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Background save of user preferences failed.");
+            SaveError = ex;
+            OnChange?.Invoke(); // Notify UI so it can show SaveError
+        }
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        _saveCts?.Cancel();
+        _saveCts?.Dispose();
+        return ValueTask.CompletedTask;
+    }
+}
+
+/// <summary>Describes a selectable DevExpress theme.</summary>
+/// <param name="Label">User-facing display name.</param>
+/// <param name="Value">CSS theme identifier (used in href and API).</param>
+public sealed record ThemeOption(string Label, string Value);
diff --git a/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/UserPreferencesService.cs b/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/UserPreferencesService.cs
new file mode 100644
index 00000000..bad20009
--- /dev/null
+++ b/EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/UserPreferencesService.cs
@@ -0,0 +1,130 @@
+using System.Net.Http.Json;
+using System.Text.Json;
+using EnvelopeGenerator.Application.Common.Dto;
+using Microsoft.Extensions.Logging;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+/// <summary>
+/// Retrieves and persists user UI preferences via the server API.
+/// The server stores preferences in the distributed cache (dbo.TBDD_CACHE).
+///
+/// Client-side memory cache:
+///   Preferences are cached in-memory for <see cref="CacheDuration"/> after the first fetch.
+///   All reads (GetAsync, GetCachedAsync) are served from cache after the first load.
+///   Writes (SaveAsync) update both the cache and the server atomically.
+///   Cache is invalidated explicitly via InvalidateCache().
+/// </summary>
+public class UserPreferencesService(
+    IHttpClientFactory clientFactory,
+    ILogger<UserPreferencesService> logger)
+{
+    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
+    private const string Endpoint = "/api/UserPreferences";
+
+    // ── Client-side memory cache ──────────────────────────────────────────────
+    private static readonly TimeSpan CacheDuration = TimeSpan.FromHours(1);
+    private UserPreferencesDto? _cached;
+    private DateTimeOffset _cachedAt = DateTimeOffset.MinValue;
+
+    private bool IsCacheValid => _cached is not null
+        && DateTimeOffset.UtcNow - _cachedAt < CacheDuration;
+
+    /// <summary>
+    /// Returns preferences from in-memory cache if valid; otherwise fetches from the server.
+    /// On first call after login this issues one HTTP GET — all subsequent calls within
+    /// <see cref="CacheDuration"/> are served from memory with zero network overhead.
+    /// </summary>
+    public async Task<UserPreferencesDto> GetAsync(CancellationToken ct = default)
+    {
+        if (IsCacheValid)
+            return _cached!;
+
+        try
+        {
+            var client = clientFactory.CreateClient("EnvelopeGenerator.Server");
+            var response = await client.GetAsync(Endpoint, ct);
+
+            // 404 means no preferences saved yet — return and cache defaults.
+            if (response.StatusCode == System.Net.HttpStatusCode.NotFound)
+            {
+                SetCache(new UserPreferencesDto());
+                return _cached!;
+            }
+
+            response.EnsureSuccessStatusCode();
+
+            var dto = await response.Content.ReadFromJsonAsync<UserPreferencesDto>(_jsonOptions, ct)
+                ?? new UserPreferencesDto();
+
+            SetCache(dto);
+            return _cached!;
+        }
+        catch (Exception ex) when (ex is not TaskCanceledException)
+        {
+            logger.LogError(ex, "Failed to load user preferences from server.");
+            throw;
+        }
+    }
+
+    /// <summary>
+    /// Returns the cached preferences without hitting the network.
+    /// Returns null if the cache has not been populated yet (before first GetAsync call).
+    /// </summary>
+    public UserPreferencesDto? GetCached() => IsCacheValid ? _cached : null;
+
+    /// <summary>
+    /// Persists the given preferences on the server and updates the in-memory cache.
+    /// </summary>
+    public async Task<UserPreferencesDto> SaveAsync(UserPreferencesDto preferences, CancellationToken ct = default)
+    {
+        try
+        {
+            var client = clientFactory.CreateClient("EnvelopeGenerator.Server");
+            var response = await client.PutAsJsonAsync(Endpoint, preferences, _jsonOptions, ct);
+            response.EnsureSuccessStatusCode();
+
+            var saved = await response.Content.ReadFromJsonAsync<UserPreferencesDto>(_jsonOptions, ct)
+                ?? preferences;
+
+            SetCache(saved);
+            return saved;
+        }
+        catch (Exception ex) when (ex is not TaskCanceledException)
+        {
+            logger.LogError(ex, "Failed to save user preferences to server.");
+            throw;
+        }
+    }
+
+    /// <summary>
+    /// Updates a single grid layout key without a network read.
+    /// Uses the in-memory cache for the current state, merges the new layout, then saves.
+    /// If the cache is empty (cold start), fetches from server first.
+    /// </summary>
+    /// <param name="gridKey">Stable grid identifier, e.g. "sender.active-envelopes".</param>
+    /// <param name="layoutJson">Opaque layout JSON produced by DxGrid.SaveLayoutAsync().</param>
+    public async Task SaveGridLayoutAsync(string gridKey, string layoutJson, CancellationToken ct = default)
+    {
+        // Use cached copy — avoids a GET before every grid layout save.
+        var prefs = GetCached() ?? await GetAsync(ct);
+        prefs.GridLayouts[gridKey] = layoutJson;
+        await SaveAsync(prefs, ct);
+    }
+
+    /// <summary>
+    /// Invalidates the in-memory cache, forcing the next GetAsync to fetch from the server.
+    /// Call this after logout or when a forced refresh is needed.
+    /// </summary>
+    public void InvalidateCache()
+    {
+        _cached = null;
+        _cachedAt = DateTimeOffset.MinValue;
+    }
+
+    private void SetCache(UserPreferencesDto dto)
+    {
+        _cached = dto;
+        _cachedAt = DateTimeOffset.UtcNow;
+    }
+}