Struggle to do design for your pet project? No worries, there are already websites that will help you with economy your time and focus fully on developing!
One of the most popular websites with free and paid designs is frontend mentor. You can search by stack and your level. Advanced filtering and modern designs.
ThemeForest is also a good option, but for the paid designs only. It mostly focuses on landing pages. Good source to develop real designs you may face in the future!
DailyUI - new frontend challenges every day. Moreover, it's for free! All is needed is to sign up.