Fixes #11933

`ToolStripRenderer.OnRenderItemCheck` only inverted the check mark glyph when `SystemInformation.HighContrast` was true. In regular dark mode the check mark icons (designed for light backgrounds) were drawn as-is, resulting in a dark glyph on a dark background.

Extended the inversion logic in `ToolStripRenderer.OnRenderItemCheck` to also trigger when `Application.IsDarkModeEnabled` is true, using the existing `ControlPaint.CreateBitmapWithInvertedForeColor` helper. The high contrast path is preserved as a separate branch since it already handles its own background-color detection.

Indeterminate and checked `ToolStripMenuItem` icons are now clearly visible in dark mode on `ContextMenuStrip`, `MenuStrip`, `StatusStrip` and `ToolStrip` drop-down buttons.

No

Minimal

Manual

- 11.0.100-preview.1.26078.121

Author: Ricardo Bossan (BEYONDSOFT CONSULTING INC) (from Dev Box)

src/System.Windows.Forms/System/Windows/Forms/Controls/ToolStrips/ToolStripRenderer.cs

@@ -863,14 +863,22 @@ protected virtual void OnRenderItemCheck(ToolStripItemImageRenderEventArgs e)
                 image = CreateDisabledImage(image, e.ImageAttributes);
             }
 
-            if (SystemInformation.HighContrast && image is Bitmap bitmap)
+            if (image is Bitmap bitmap)
             {
-                Color backgroundColor = e.Item.Selected ? SystemColors.Highlight : e.Item.BackColor;
-
-                if (ControlPaint.IsDark(backgroundColor))
+                if (Application.IsDarkModeEnabled)
+                {
+                    // In dark mode, the check mark icons are dark glyphs designed for light
+                    // backgrounds. Invert the foreground color so they are visible on dark backgrounds.
+                    image = ControlPaint.CreateBitmapWithInvertedForeColor(bitmap, e.Item.BackColor);
+                }
+                else if (SystemInformation.HighContrast)
                 {
-                    Image invertedImage = ControlPaint.CreateBitmapWithInvertedForeColor(bitmap, e.Item.BackColor);
-                    image = invertedImage;
+                    Color backgroundColor = e.Item.Selected ? SystemColors.Highlight : e.Item.BackColor;
+
+                    if (ControlPaint.IsDark(backgroundColor))
+                    {
+                        image = ControlPaint.CreateBitmapWithInvertedForeColor(bitmap, e.Item.BackColor);
+                    }
                 }
             }
         }