Support a way to set the primary color of a site

[damithc]

Inspired by https://yakworks.github.io/mkdocs-material-components/getting-started/#primary-colors

[tlylt]

Inspired by yakworks.github.io/mkdocs-material-components/getting-started/#primary-colors

the above link is dead... it's now available at this link instead.
Screenshot:

[ang-zeyu]

This won't be accomplished anytime soon, but to list the key issues in the various linked discussions above:

• bootstrap themes seem to conflict with this both from an implementation and usage standpoint.
• I'm personally for removing official support for the 15+ bootstrap themes, and developing our components with bootstrap, which have proven very difficult to maintain properly (there are many, many issues if you dig around).
• (personally) everything looks very bootstrap-ey
• that said, implementing a completely custom theming scheme isn't easy, I haven't exactly thought of a concrete proposal either (which might flesh out more issues =X) We should probably limit the official support to color-only (bootstrap themes do much more than that) theming for a start to make it feasible
  • A pretty solid example direction I think, is mdBook's https://github.com/rust-lang/mdBook/blob/master/src/theme/css/variables.css

wdyt @jonahtanjz @ryoarmanda @damithc?

ps: starting the discussion now as it affects #1702