Dark mode documentation has clashing colour style

[DanielArndt]

Describe the bug

The colour styling in the documentation makes it very difficult to read.

Contrast this with some similar Canonical documentation such as the Rockcraft documentation:

To Reproduce

1. Go to https://canonical-charmed-5g.readthedocs-hosted.com/en/latest/
2. If not in dark mode, enable dark mode by clicking the button in the top right
3. Scroll down to 'In this documenation'
4. See error

Expected behavior

Colour palette should make it easy to read.