Visual theme adjustments

[perlun]

This took a few evenings, but I'm quite happy about the end result now. Here's the README.md content describing these changes:

The main changes in this theme compared to darkfx are as follows:

• The light theme follows more closely the default docfx theme. For example, we specifically avoid overwriting non-color-related CSS attributes. The idea is to make this theme "lean and mean", and avoid messing with upstream styles when not absolutely necessary.

• The light theme does not use a liquid container, but instead follows the same limited-width as the default docfx theme.

• The dark theme has different colors, more resembling https://docs.microsoft.com.

• The hljs (highlightjs) colors for both the dark theme and the light theme was adjusted to use the following hljs styles:

  • https://github.com/highlightjs/highlight.js/blob/main/src/styles/vs.css (light)
  • https://github.com/highlightjs/highlight.js/blob/main/src/styles/vs2015.css (dark)

• The CSS variables were renamed to --theme-*, again similar to how docs.microsoft.com has done it.

• The CSS stylesheet was converted to SASS, which simplifies some things greatly.