Fixed the border around the languages button and the scrollbar fillers

This should fix #8389 and an issue I saw with the corner scrollbars filler in windows and both scrollbar fillers in Linux.
Yes. The color for linux is the same as the background color. Without it, it is displayed using the light bg. It works in the same way in the light theme.

To test it in windows I just replace platform-linux with platform-win and comment the windows styles.