Update the themes to match recent MkDocs changes

[jimporter]

This ports over the following PRs from MkDocs:

• mkdocs/mkdocs#1935
• mkdocs/mkdocs#1967
• mkdocs/mkdocs#1969
• mkdocs/mkdocs#1971

This should be pretty straightforward. The only real difference between all these versions of base.css is that I had to sit down and figure out exactly how tall the nav header was by default so that I could set up the offsets properly for each theme. I did that by inspecting all the CSS and just adding up the appropriate values.

I've manually tested this with every theme using every nav_style variant and it all looks ok as far as I can tell.