CSS Style code improvements

[trusktr]

This is a new issue to track the improvements to the CSS styling system. This will likely be a major version bump on its own eventually, but first we can see if there's a way we can deprecate existing style vars for a while before we remove them in a major bump.

• Prefix all Docsify CSS variables with --docsify-. For example, --theme-color should be --docsify-theme-color, or even better --docsify-accent-color.
• Deprecate old variables, but keep them around for back compat for a while.
  • We can, for example, map a new --docsify-accent-color variable to the existing --theme-color once we deprecate it, then eventually we remove --theme-color and use --docsify-accent-color directly.
  • We can write a script that checks style sheets for deprecated variables, and warns in console.
• Let's update styling code so that docsify-themeable implementation becomes simpler, if possible. (cc @jhildenbiddle)
• ...

[anniepauline]

I'd like to work on it.Is it up for grabs?