add light theme + support for switching between dark/light theme

[branfosj]

This:

• adds a light theme
• toggle selection between dark and light themes
• simplifies the dark theme to just override the changes from slate

[boegel]

Both the blue and green is a bit too light on a white background imho, so let's use slightly darker variants, see branfosj#2

[boegel]

@branfosj Looks like CodeSpell found a couple of (new) problems, let's tackle those in a separate PR?

Error: ./docs/release-notes.md:7711: extermal ==> external, extremal
Error: ./docs/release-notes.md:9147: Pytho ==> Python
Error: ./docs/release-notes.md:12488: Pilon ==> Pylon, Pillion
Error: ./docs/release-notes.md:23614: Oger ==> Ogre
Codespell found one or more problems

[branfosj]

@boegel I've fixed two spelling mistakes in 734c5cd, but I also suggest we skip spell checking the release notes (see #125)