Stylish is a free Firefox which allows you to install custom themes and skins for thousands of popular websites, delivering anything from minor tweaks (a different background colour) to a major redesign.
Getting started is as easy as visiting your target site, clicking the Stylish icon and selecting "Find styles for this site".
You'll then see a list of available styles, a text description and thumbnail preview of each, and details including its upload date, the number of downloads so far, user rating and more. (Check out userstyles.org to see what's available.)
If you like something, click it, select Install with Stylish > OK, and you're done. Revisit the page and you'll probably see the new look immediately (reload it, if not).
Repeat the process to install as many styles as you like, and, if you know what you're doing, use the Edit option to customise individual styles by editing their CSS.
This generally worked well for us, with the worst problem being that some styles didn't seem to have any effect (the sites looked exactly as they did before).
But if Stylish does cause any issues for you, it's easily disabled. Click the Stylish icon, select "Turn all styles off" and websites should now display as usual.