PR #4066 added a dark color scheme to the file_server browse template.
PR #4356 later set the links for the `:visited` pseudo-class, but did
not set anything for the dark mode, resulting in poor contrast. I
selected some new colors by feel.
This commit also adds an `a:visited:hover` for both, to go along with
the normal blue hover colors.
This makes it easier for users to find the default browse template if they
want to create a custom template based on that. It also makes it easier to
view the template with proper syntax highlighting.