The problem is that the scope of the content script is on the web page that your plugin is suppose to be used at.

So the css background:url(images/ui-bg_inset-hard_100_fcfdfd_1x100.png) becomes url('http://webpageforplugin/images/ui-bg_inset-hard_100_fcfdfd_1x100.png') in order for this to work as far as i understood i need to have it to point to: url('chrome-extension://extensionId/images/ui-bg_inset-hard_100_fcfdfd_1x100.png')

So i tried to haxorz the document.styleSheets

var ss = document.styleSheets;

for (var i=0; i<ss.length; i++) {
    var found=-1, x,i;
    var rules = ss[i].cssRules || ss[i].rules;

    for (var j=0; j<rules.length; j++) {
        if ('.ui-helper-hidden'==rules[j].selectorText){
            found=i;
            break;
        }
    }
    if (found>-1){
        for (var j=0; j<rules.length; j++) {
            if (x=rules[j].style.background){
                if ((i=x.indexOf('url'))!=-1)
                    rules[j].style.background = x.replace('http://page/images/','chrome-extension://extensionId/images/');
            }
        }
        break;
    }   
};

I feel that i'm missing the obvious. That there must be an easier way.

Even if i manage to change this how will i get the extension id to build the string.

Btw this doesn't work, the icons are not properly fetched. (I hardcoded the extension id)

Any ideas?