Monochrome version of themes

[ascarter]

Description

For people that have color blindness, the use of color can be problematic. A nice option would be to have a more subdued palette that uses monochrome contrast instead of colors. Additionally, syntax highlighting with fewer color variations may be generally preferred as well. A variation of the Light/Dark/Dimmed themes with fewer colors used or a full monochrome (just using levels of gray/black/white) would be a useful choice.

Steps to reproduce

See this issue as an example of color issues
#352

Version

v6.3.3

Theme

GitHub Dark Dimmed