Introduce a DxComboBox in MainLayout for selecting between multiple themes. Update ThemeState to manage the current theme, provide a list of available themes, and apply the selected theme via a new SetTheme method. Refactor dark mode handling to work with the new theme system, and ensure UI updates on theme or mode changes.
Implement dark mode support using a new ThemeState service that manages theme switching and integrates with DevExpress Blazor theming. Update App.razor to apply the theme globally, enhance MainLayout with a toggle button and dynamic CSS classes, and add dark mode styles to CSS files. Register ThemeState as a scoped service in Program.cs.
