> One thing that has a problem, though: we have some
> dynamic images in our css files, like
> "url(/media.php?path=/static_images/nav2-bg.gif)".
> These were downloaded, but NOT replaced in the CSS
> file. CSS file still says "background: #0092dd

Can you provide a reproductible example ?
I tried with a minimal css file example:
.foo-bar        {  foo: url(media.php?path=/static_images/nav2-bg.gif); bar:
bar}

And it worked.

The problem is probably related to more complicated css code, or a parsing
error somewhere.

Can you also enable "debug" log, and see if the css file is being parsed
("scanning file .." lines in hts-log.tct)