Style editor scrollbars with light/dark ui theme

Make scrollbars not as jarring by styling them according to the current
ui theme's background and foreground colors
Improve scrollbars styling

Correctly place the border depending on whether the border is vertical
or horizontal. Also made the thumb colors a bit less pronounced
Move scrollbar styling to ui.less
Remove scrollbar track border
Make scrollbar track background transparent
Merge pull request #100 from elisee/better-scrollbars

Style editor scrollbars with light/dark ui theme