90%
Don't write CSS, Compose Classes
The Design System is extremely flexible and powerful. 90% of interface solutions can be composed from the base utility classes, making our CSS small and DRY, and avoids the common pattern of authoring more and more CSS per feature.
8%
Add new classes to Libraries
If you find it's not serving your needs, then the next step is to contribute a reusable utility to the library. This way, other teams can use it in their project and get the benefit of your solutions.
2%
Write Custom CSS
Finally, you can write custom CSS per project. It's strongly suggested you try to solve the problem with this system first. This reduces repetion of code/work, reduces hard to replicate bugs, and increases consistency of the Fender brand/design language.