Since AO3 no longer allows users to make their themes public, you'll have to copy the CSS code and paste it in the "Create Site Skin" section. Go to AO3 and log in > "My Dashboard" > "Skins" > "Create ...

Selecting a CSS template is a bit like selecting an outfit do it right and you feel good about yourself, you look great people are impressed. Choose badly and your site will either be inflexible, ...