This PR adds colorblind theme variants of the forgejo themes. I duplicated the forjego light and dark themes and only changed the lines related to diff colors for added and removed rows/words.
I am not a designer, and I am also colorblind, so better suggestions of colors are most welcome. However, this is a good start as I can at least personally see the colors now. I got the colors for the dark theme from the GitHub diff colors, the light ones I couldn't get from GitHub as they use white as a plain background, which Forgejo's theme doesn't, so they were decided on after a bit of random testing.
Resolves #986
(cherry picked from commit dcdb4a372d)
