The backgroundColor property is part of the “required” schema of the object, and so even though you can delete it in the Designer, the component is still built with all of its required properties each time it loads. Fortunately, there’s a workaround: anywhere you have a backgroundColor property you want to overwrite/remove, replace the value of backgroundColor with null. This sort of short-circuits the property into being invalid, and so it falls back to the previous valid value, which in your case is your style.
You’ll see a warning in the Designer, but you can safely ignore it.
