fix: fix state sync.

src/Ursa.Themes.Semi/Controls/ThemeSelector.axaml

@@ -8,6 +8,7 @@
                 <Button
                     Padding="8"
                     Name="{x:Static u:ThemeToggleButton.PART_ThemeButton}"
+                    HorizontalAlignment="Center"
                     Theme="{DynamicResource BorderlessButton}">
                     <PathIcon
                         Name="PART_Icon"

src/Ursa/Controls/ThemeSelector/ThemeToggleButton.cs

@@ -90,6 +90,9 @@ public class ThemeToggleButton: ThemeSelectorBase
             PseudoClasses.Set(PC_Light, theme == ThemeVariant.Light);
             PseudoClasses.Set(PC_Dark, theme == ThemeVariant.Dark);
             PseudoClasses.Set(PC_Default, theme == null || SelectedTheme == ThemeVariant.Default);
+            if (theme == ThemeVariant.Dark) _state = false;
+            else if (theme == ThemeVariant.Light) _state = true;
+            else _state = null;
         }
     }
 }