Merge pull request AvaloniaUI#8291 from AvaloniaUI/fix-fluent-loader

Fix FluentTheme density style

Author: Takoooooo

src/Avalonia.Themes.Fluent/FluentTheme.cs

@@ -81,6 +81,14 @@ public DensityStyle DensityStyle
         protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
         {
             base.OnPropertyChanged(change);
+            
+            if (_loaded is null)
+            {
+                // If style wasn't yet loaded, no need to change children styles,
+                // it will be applied later in Loaded getter.
+                return;
+            }
+            
             if (change.Property == ModeProperty)
             {
                 if (Mode == FluentThemeMode.Dark)