Improve SolidColorBrush usage and property checks in Theme

Refactor background brush creation to assign, freeze, and reuse
SolidColorBrush instances for better WPF performance. Replace
string-based property checks with type-safe Control.BackgroundProperty
comparisons for improved robustness.

Author: Jack251970

Flow.Launcher.Core/Resource/Theme.cs

@@ -673,13 +673,17 @@ private void SetBlurForWindow(string theme, BackdropTypes backdropType)
                 // If the BackdropType is Mica or MicaAlt, set the windowborderstyle's background to transparent
                 if (backdropType == BackdropTypes.Mica || backdropType == BackdropTypes.MicaAlt)
                 {
-                    windowBorderStyle.Setters.Remove(windowBorderStyle.Setters.OfType<Setter>().FirstOrDefault(x => x.Property.Name == "Background"));
-                    windowBorderStyle.Setters.Add(new Setter(Border.BackgroundProperty, new SolidColorBrush(Color.FromArgb(1, 0, 0, 0))));
+                    windowBorderStyle.Setters.Remove(windowBorderStyle.Setters.OfType<Setter>().FirstOrDefault(x => x.Property == Control.BackgroundProperty));
+                    var brush = new SolidColorBrush(Color.FromArgb(1, 0, 0, 0));
+                    brush.Freeze();
+                    windowBorderStyle.Setters.Add(new Setter(Border.BackgroundProperty, brush));
                 }
                 else if (backdropType == BackdropTypes.Acrylic)
                 {
-                    windowBorderStyle.Setters.Remove(windowBorderStyle.Setters.OfType<Setter>().FirstOrDefault(x => x.Property.Name == "Background"));
-                    windowBorderStyle.Setters.Add(new Setter(Border.BackgroundProperty, new SolidColorBrush(Colors.Transparent)));
+                    windowBorderStyle.Setters.Remove(windowBorderStyle.Setters.OfType<Setter>().FirstOrDefault(x => x.Property == Control.BackgroundProperty));
+                    var brush = new SolidColorBrush(Colors.Transparent);
+                    brush.Freeze();
+                    windowBorderStyle.Setters.Add(new Setter(Border.BackgroundProperty, brush));
                 }
 
                 // For themes with blur enabled, the window border is rendered by the system, so it's treated as a simple rectangle regardless of thickness.
@@ -803,7 +807,9 @@ private void ApplyPreviewBackground(Color? bgColor = null)
 
             // Apply background color (remove transparency in color)
             Color backgroundColor = Color.FromRgb(bgColor.Value.R, bgColor.Value.G, bgColor.Value.B);
-            previewStyle.Setters.Add(new Setter(Border.BackgroundProperty, new SolidColorBrush(backgroundColor)));
+            var brush = new SolidColorBrush(backgroundColor);
+            brush.Freeze();
+            previewStyle.Setters.Add(new Setter(Border.BackgroundProperty, brush));
 
             // The blur theme keeps the corner round fixed (applying DWM code to modify it causes rendering issues).
             // The non-blur theme retains the previously set WindowBorderStyle.
@@ -917,11 +923,15 @@ private void ColorizeWindow(string theme, BackdropTypes backdropType)
                 // Only set the background to transparent if the theme supports blur
                 if (backdropType == BackdropTypes.Mica || backdropType == BackdropTypes.MicaAlt)
                 {
-                    mainWindow.Background = new SolidColorBrush(Color.FromArgb(1, 0, 0, 0));
+                    var backgroundBrush = new SolidColorBrush(Color.FromArgb(1, 0, 0, 0));
+                    backgroundBrush.Freeze();
+                    mainWindow.Background = backgroundBrush;
                 }
                 else
                 {
-                    mainWindow.Background = new SolidColorBrush(selectedBG);
+                    var backgroundBrush = new SolidColorBrush(selectedBG);
+                    backgroundBrush.Freeze();
+                    mainWindow.Background = backgroundBrush;
                 }
             }
         }