cssRules/rules are null in Chrome
Content scripts don't have any cross-domain privileges comparing to a regular javascript, so any limitations are carried over. See related question #1, question #2.
You can inject your own css style in the manifest:
"content_scripts": [ { "matches": ["http://www.google.com/*"], "css": ["mystyles.css"] }]where you can try to overwrite original styles by defining rules with higher specificity.
You can also just tweak concrete element styles through javascript:
document.getElementById("id").style.property="value";
I fixed my version of the issue by changing the url from http:// to https://. Doh!