If you have access to jQuery you could do this pretty easily:
var css_href = "path to css";
var head = document.getElementsByTagName('head')[0];
$(document.createElement('link'))
.attr({ type: 'text/css',
href: css_href,
rel: 'stylesheet,
media: 'screen''})
.appendTo(head);
[Source]
This can also be done pretty easily with vanilla JS:
function addStyle(style) {
var head = document.getElementsByTagName("HEAD")[0];
var ele = head.appendChild(window.document.createElement( 'style' ));
ele.innerHTML = style;
return ele;
}
addStyle('@import "/URL/TO/STYLESHEET;"');
[Source] (Apparently from "Dive into greasemonkey" originally)
You might want to make sure your styles have !important
declarations though.
Having said that, it should be easy to remove the existing stylesheets this way too and perhaps even iterate elements and remove inline styles.