[File Explorer Add-ons] Fix file preview pane flickering on file selection and resizing (#26660)

* Move color values to constant

* Fix MonacoPreview flickering on file selection and resizing

* Fix MarkdownPreview flickering on file selection and resizing

* Fix SvgPreview flickering on file selection and resizing

* Create Settings class and standardize background setting for MarkdownPreview

* Replace ColorTranslator.FromHtml with Color.FromArgb for constant color settings

* Use existing SetBackground

* Remove duplicate GetTheme function

* Update src/modules/previewpane/MarkdownPreviewHandler/Settings.cs

src/modules/previewpane/MonacoPreviewHandler/MonacoPreviewHandlerControl.cs

@@ -77,6 +77,11 @@ namespace Microsoft.PowerToys.PreviewHandler.Monaco
         /// </summary>
         private string _base64FileCode;
 
+        public MonacoPreviewHandlerControl()
+        {
+            this.SetBackground();
+        }
+
         [STAThread]
         public override void DoPreview<T>(T dataSource)
         {
@@ -95,14 +100,12 @@ namespace Microsoft.PowerToys.PreviewHandler.Monaco
 
             base.DoPreview(dataSource);
 
-            // Sets background color
-            SetBackground();
-
             // Starts loading screen
             InitializeLoadingScreen();
 
             // New webview2 element
             _webView = new WebView2();
+            _webView.DefaultBackgroundColor = Color.Transparent;
 
             // Checks if dataSource is a string
             if (!(dataSource is string filePath))

src/modules/previewpane/MonacoPreviewHandler/Settings.cs

@@ -86,7 +86,7 @@ namespace Microsoft.PowerToys.PreviewHandler.Monaco
             {
                 if (GetTheme() == "dark")
                 {
-                    return System.Drawing.ColorTranslator.FromHtml("#1e1e1e");
+                    return Color.FromArgb(30, 30, 30); // #1e1e1e
                 }
                 else
                 {