Make every theme setting reachable and the theme controls effective

With the default settings (follow the system, light grey) the nav's hue slider
and dark toggle did nothing visible, and no UI could leave that mode. The slider
also always wrote the light hue, even in dark mode.

PageSettings gains SetDarkMode, SetGray and SetHue, and CascadingState gains
UpdatePageSettings. They keep the flags consistent:
- choosing light or dark leaves system theming;
- picking a hue leaves grey;
- grey follows the variant shown.
theme.js reports which variant is showing (AppStatusService.IsDarkTheme), so the
nav toggle and slider edit that one, including the OS's choice in system mode.

Settings > General, which was empty, now edits every theme setting:
- follow the system;
- dark mode;
- grey;
- the light and dark hues.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Pages/Settings.razor

@@ -6,17 +6,33 @@
 
 	<div class="flex flex-col gap-4 p-4 md:p-5 w-full h-full absolute overflow-y-auto">
 
-		<DCollapse Icon="DCollapseIcon.Arrow">
+		<DCollapse Icon="DCollapseIcon.Arrow" IsOpen="true">
 			<Header>
 				<p class="inline-flex items-center gap-2">
 					<i class="ion-md-cog text-xl"></i> <span>@Localizer["General"]</span>
 				</p>
 			</Header>
 			<ChildContent>
-				<div class="block w-full p-3 md:p-4">
-					<p class="w-full text-center text-lg">
-						<i class="ion-ios-remove-circle-outline"></i> @Localizer["Empty"]
-					</p>
+				@* Every theme setting; NavMenu's toggle and slider are shortcuts to the dark and hue ones. *@
+				<div class="flex flex-col gap-4 w-full p-3 md:p-4">
+					<DToggle LabelText="@Localizer["Follow the system's light or dark setting"]" Value="Theme.PreferSystemTheming"
+							 ValueChanged="v => Update(s => s.PreferSystemTheming = v)" ValueExpression="() => Theme.PreferSystemTheming" />
+					<DToggle LabelText="@Localizer["Dark mode"]" Value="IsDark"
+							 ValueChanged="v => Update(s => s.SetDarkMode(v))" ValueExpression="() => IsDark" />
+					<DToggle LabelText="@Localizer["Grey"]" Value="IsGray"
+							 ValueChanged="v => Update(s => s.SetGray(v, IsDark))" ValueExpression="() => IsGray" />
+					<label class="flex flex-col gap-2 max-w-md">
+						<span class="text-sm">@Localizer["Light theme colour"]</span>
+						<DRange TValue="short" class="neo-hue-track w-full" Size="DSize.Xs" Min="0" Max="359" Step="1"
+								Value="Theme.LightThemeIndexColour" ValueChanged="v => Update(s => s.SetHue(false, v))"
+								ValueExpression="() => Theme.LightThemeIndexColour" />
+					</label>
+					<label class="flex flex-col gap-2 max-w-md">
+						<span class="text-sm">@Localizer["Dark theme colour"]</span>
+						<DRange TValue="short" class="neo-hue-track w-full" Size="DSize.Xs" Min="0" Max="359" Step="1"
+								Value="Theme.DarkThemeIndexColour" ValueChanged="v => Update(s => s.SetHue(true, v))"
+								ValueExpression="() => Theme.DarkThemeIndexColour" />
+					</label>
 				</div>
 			</ChildContent>
 		</DCollapse>
@@ -72,4 +88,10 @@
 
 @code {
 	[CascadingParameter] CascadingState CascadingState { get; set; }
+
+	PageSettings Theme => CascadingState.PublicCacheData?.PageSettings ?? new();
+	bool IsDark => CascadingState.Status.IsDarkTheme();
+	bool IsGray => Theme.IsGray();
+
+	Task Update(Action<PageSettings> change) => CascadingState.UpdatePageSettings(change).AsTask();
 }