Anyone understand why a theme's css files are "cached"?

From what I can tell, the files are copied into a folder /files/cache/css/{theme-name}/css/{stylesheet}.css.

Each URL reference in a file, e.g.

background: url('images/bg.png')

is replaced by a full URL:

background: url('http://domain.com/themes/{theme-name}/images/bg.png')

The problem here is that a page loaded using SSL will get "unprotected resources" messages because it includes this style sheet that now has hard-coded http references.

Anyone found a good work-around for this issue?