feature/dev 1879 improve contrast on certain elements

[stanley2058]

Improve contrast of UI elements:

• Default theme now has higher contrast linenumber & code comments.
  The link buttons in the navbar, last changed at, and the action button on the top right are now more visible in dark mode.
• Fix an issue where the theme name would not be visible when hovering under dark mode.

Before:

After:

The color of active line is also updated:
(Before)

(After)