Document the themes that respect WCAG 2.1 AA

[quentinderoubaix]

Currently, no light theme in the provided list respects the color contrasts of WCAG 2.1 AA.
Thus, a website trying to have good accessibility always needs to use a custom theme or change the colors of default themes.

(Optimistic) Proposal:
Document in the README under https://github.com/shikijs/textmate-grammars-themes/tree/main/packages/tm-themes all the themes that respect WCAG 2.1 AA, so that users may choose an accessible theme, without having to check themselves.

[antfu]

I don't have the knowledge of how to do it. PR welcome if anyone is interested.