Improve hint marker styling

Improve hint marker styling

- Increase contrast between the hint chars and the background color by
  changing the text color to fully black.

- Make the styling for hint markers for scrollable elements scale with
  the font size.

- Make highlighted hint markers more prominent. The changes in commit
  4bb0b56c made the difference _too_ subtle. While it looked pretty good
  on the computer I developed it on, the highlighting was almost
  invisible on a different screen. The new styling relies not solely on
  color, but also on border-radius, to increase the visual difference.