Stylish allows you to style the interface of any site when help with user-created themes.
Most popular sites already have a list of user-created themes. For example, WhatsApp, GitHub, Instagram, Google, Stackoverflow, etc.
To add a theme, you need:
one. Install Stylish.
2. Go to the site of interest.
3. Click on the addon icon.
4. Follow the link “Find more styles for this site”.
5. Choose the interface style you like.
6. Press “Install Style”.
It is possible to set several interface styles at the same time. For example, the themes “Fade++” and “Nyan Cat”.