Go all-in on CSS custom properties for themes, remove Sass variables

[craigcook]

Much of the preliminary work we've done in bedrock for the new Mozilla branding has hinged on custom properties and eschewed Sass variables. And frankly it simplified SO MUCH. It makes theme-switching and color modes much easier to implement without worrying about Sass compile errors or importing things in the right order.

We semi-recently converted Protocol to use custom properties for theming but preserved the Sass variables as fallbacks for older browsers. But looking at support levels and browser usage stats, I've been thinking it's time to embrace the future of CSS and remove those legacy Sass variables (at least for all the theme-level properties; some things can/should remain as Sass variables and mixins).

[kkellydesign]

I just took a stab at this ^^

There's still some work to be done. (I'm going to tackle the font mixins next, #998), but I don't want the PR to be too hard to follow.

[stephaniehobson]

@kkellydesign Please ask before taking on tasks that have been assigned to people.