Clean up CSS, make the UI appear in Firefox

The main container of the UI never faded its opacity in Firefox, which made it seem like it was doing nothing. Defining keyframe animations using the standards as well as the `-webkit` prefix makes it work in both Chrome and in Firefox.

Remove several HTTP requests by copying the contents of Google's font stylesheets into `marathon.css`.

Remove unused 900-weight version of Lato font. It is only referenced in 100- and 300 weight.

Other changes are documented in the commits.