self check (web based): Support custom selfcheck.css file

EG 2.10

It would be nice if the web based self check supported a custom css file, so local modifications would never be overwritten.

One method that I've seen for doing this is to use an @import command at the beginning of the current selfcheck.css file. But I don't really like this method since the declarations in the selfcheck.css get read after the custom ones, and have precedence. So it makes it hard to override them.

Another method would be to have another link in the main.tt2 file to a custom css file.
<link rel='stylesheet' type='text/css' href='[% ctx.media_prefix %]/css/skin/[% ctx.skin %]/selfcheck.css'/>
<link rel='stylesheet' type='text/css' href='[% ctx.media_prefix %]/css/skin/[% ctx.skin %]/selfcheck_custom.css'/>

Is there any way to get around having links to non-existent files though, to avoid generating 404s.

Josh