High contrast color scheme option

[vsoch]

Hi there! We are deploying this template (love it btw, thank you!) to some of our sites, and are worried that the colors are a bit light. We see there is an issue open for a dark mode #89 (which would be amazing) and we are also wondering if there could be modes / options for higher contrast colors? E.g., here is an example from my group to compare traditional sphinx to the defaults here:

vs.

A lot could be alleviated by the dark mode, but we were thinking a higher contrast mode could be useful for us (and likely others that look at web accessibility). Thank you!