Closes #168 -- for real this time

Commit bc315d9 made changes to the CSS of VimFx:

1. It changed from "agent" sheets to "author" sheets
2. It removed all `!important`

That caused problems with userstyles in userContent.css or Stylish.
Website themes installed by the user often make the hint markers
unreadable, for example.

Commit 21a09a1 reverted #2.

After a while, though, I noticed that it wasn't enough, unfortunately.

This commit reverts #1, and adds a comment about it.
Merge pull request #179 from lydell/userstyles-fix

Closes #168 -- for real this time