Update CreatePopupItem method

-Modify the `TextColour` property to use a dynamic value based on `TextColour` multiplied by `0.9f` instead of a fixed value (`Colour.White * 0.9f`).
-Modify the `TextColourHighlighted` property to use the dynamic value of `TextColourHighlighted` instead of a fixed value (`Colour.White`).

Source/Engine/UI/GUI/Common/Dropdown.cs

@@ -594,8 +594,8 @@ namespace FlaxEngine.GUI
                 X = margin,
                 Size = new Float2(size.X - margin, size.Y),
                 Font = Font,
-                TextColor = Color.White * 0.9f,
-                TextColorHighlighted = Color.White,
+                TextColor = TextColor * 0.9f,
+                TextColorHighlighted = TextColorHighlighted,
                 HorizontalAlignment = HorizontalAlignment,
                 VerticalAlignment = VerticalAlignment,
                 Text = _items[i],