Improve dark theme

Improve dark theme color palette

Use colors (primary, secondary, warn, warn-soft) of particify light theme with less saturation.

Also extend theme mixin to pick colors according to current theme.

Furthermore switch to black font in access right settings entry where primary variant is used.

Improve background, surface and dialog colors in dark mode

Use brighter colors to provide a wider range of color, elevation, and depth.

These colours should also fit better with other dark themes of browsers and OSs.

Comparision:

Previosly (left) and now (right)

Closes #1061 (closed)