Convert Stylus to CSS

[mattico]

A revival of #636.

Moving from Stylus has been discussed before, but to summarize:

• Removes the need for node, npm, stylus, nib, and build.rs. This makes it easier to build and contribute, and also significantly improves build times.
• Removes the confusion for users and contributors who don't notice that book.css isn't the real source for styling, or who don't understand Stylus syntax.
• Makes it easy to provide an API for user-created CSS themes.
• Using CSS variables for themes makes the output CSS much smaller due to reduced duplication (though we could use those with Stylus as well).

Here's what this PR does:

• Convert Stylus to CSS (manually).
• Remove Stylus supporting code and documentation.
• Merge the many CSS files into 4 files.
  Having lots of small files is annoying to deal with in our current theme implementation (fixing that is
  another discussion), and it's bad for page load times. While we could use a build step to concatenate
  it's much simpler to not, and we don't actually have enough CSS to need many files. We could easily
  just have one stylesheet, but I liked having a little bit of organization.