Fixed the border around the languages button and the scrollbar fillers

[TomMalbran]

This should fix #8389 and an issue I saw with the corner scrollbars filler in windows and both scrollbar fillers in Linux.

[redmunds]

I verified that this fixes problem on Windows and doesn't break scrollbar or gutter fillers on Mac. Did not test Linux.

All tests pass (except for known issue 8519)).

Style changes look good, but this may conflict with changes @dangoor is working on, so I'm going to hold off merging. Kevin?

cc @MiguelCastillo

[dangoor]

Is it intentional that the linux and windows colors are different?

[TomMalbran]

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.

[dangoor]

I was away for the past few hours and it turned out that I haven't had a chance to make any changes yet... which means I have no conflicts to worry about, so I can merge this and work from here.

[TomMalbran]

@dangoor I added another fix. The line highlight was white before the code when the line-numbers are off.

[dangoor]

Good catch. I tried the line number fix and it works fine. Randy tested the other fixes. Merging.