Fix CSS for dark vs light mode

[shybovycha]

Change the CSS overrides to rely on the existing Doxygen CSS variables and user-preferred theme (light or dark mode) instead of hard-coded dark-mode-only values.

Currently the docs pages only look good in dark mode. If the users prefer light mode, the docs are very hard to consume.

Doxygen actually already comes with viable values for the light mode. In order to properly support both, added an override for these variables for the dark mode and remove the hard-coded values wherever plausible.

Screenshots

Before - light mode

Before - dark mode

After - light mode

After - dark mode