Comment 33 for bug 747197

Adam, we _do_ have such a way. The CSSOM should allow you to walk the
document.styleSheets array, walk the .cssRules array for each sheet, check
whether a rule has a background image set in it, change the URL if so, save the
.cssText for each rule, recurse down @import rules, etc. I can't actually
think of any bugs we have blocking that sequence of actions.... If you _do_ run
into bugs in that code that block this, please let me know.