private Color calculateActiveBorderColor() {
			if( !colorizationAffectsBorders )
				return null;

			Color colorizationColor = getColorizationColor();
			if( colorizationColor != null ) {
				int colorizationColorBalance = getColorizationColorBalance();
				if( colorizationColorBalance < 0 || colorizationColorBalance > 100 )
					colorizationColorBalance = 100;

				if( colorizationColorBalance == 0 )
					return new Color( 0xD9D9D9 );
				if( colorizationColorBalance == 100 )
					return colorizationColor;

				float alpha = colorizationColorBalance / 100.0f;
				float remainder = 1 - alpha;
				int r = Math.round( colorizationColor.getRed() * alpha + 0xD9 * remainder );
				int g = Math.round( colorizationColor.getGreen() * alpha + 0xD9 * remainder );
				int b = Math.round( colorizationColor.getBlue() * alpha + 0xD9 * remainder );

				// avoid potential IllegalArgumentException in Color constructor
				r = Math.min( Math.max( r, 0 ), 255 );
				g = Math.min( Math.max( g, 0 ), 255 );
				b = Math.min( Math.max( b, 0 ), 255 );

				return new Color( r, g, b );
			}

			Color activeBorderColor = (Color) Toolkit.getDefaultToolkit().getDesktopProperty( "win.frame.activeBorderColor" );
			return (activeBorderColor != null) ? activeBorderColor : UIManager.getColor( "MenuBar.borderColor" );
		}
Window top border used on Windows 10.
No longer needed since Windows 11.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatNativeWindowBorder.java