Simple addon. It's very intuitive and easy to use.
For those who complain about the impossibility to save your customonization (for another device or anything else) , it's because of security. It's quite complicated for new users to deal with that.
BUT THERE IS AN ALTERNATIVE : just save or bookmark the URL when you set your colors (it changes after ANY modification you make). After that, you just need to revisit your saved URL to reinstall your custom colors.