Improve spacing for Dark Mode button in top row

Wrapped the Dark Mode toggle button in a span with left margin
for better separation from the theme combo box. Added a new
.btn-gap CSS class to standardize button spacing in the top row.

DbFirst.BlazorWebApp/Components/Layout/MainLayout.razor

@@ -13,8 +13,10 @@
                         Value="@ThemeState.CurrentThemeName"
                         ValueChanged="@((string t) => ThemeState.SetTheme(t))"
                         style="width: 130px;" />
+            <span style="margin-left: 12px;">
                 <DxButton Text="@(ThemeState.IsDarkMode ? "Dark Mode aus" : "Dark Mode an")"
                           Click="ToggleTheme" />
+            </span>
             <a href="https://learn.microsoft.com/aspnet/core/" target="_blank">About</a>
         </div>
 

DbFirst.BlazorWebApp/wwwroot/app.css

@@ -185,3 +185,7 @@ dxbl-grid tbody tr:nth-child(even) td {
     align-items: center;
     justify-content: center;
 }
+
+.top-row .btn-gap {
+    margin-left: 8px;
+}