Clean up CSS/LESS
The style sheets have grown quite a bit over time, but AFAIK have never been pruned.
For example, we removed a template override in the v9 migration whose only purpose was to specify an additional filename
class in the file list box.
As such, this rule won't be applied anymore.
We should go through the style sheets and them up a bit. While we're at it, we should also add some comments on opaque rules.