It depends which browser you’re using. If it’s Firefox you can install the add-on Stylish, then with that add-on create a new stylesheet (“Write new style” under the Stylish category in the Add-ons Manager), insert HTML namespace as default (using the “Insert…” button at the top of the Stylish editor), and put that code in the new stylesheet. If it’s a different browser then I don’t know, but I’d be willing to do some googling and try to find out.
