Do you use a lightweight style guide or a full component library for weekend builds?

It's so easy to fall into the trap of over engineering a quick weekend project just to keep the styling under control. A massive design system feels like total overkill when you're just testing an idea but without some rules the third screen always looks like a completely different app. I've started keeping a basic note with just a few color and spacing constraints to feed the assistant.

What is your go to trick to keep your UI looking clean without slowing down your build speed?