We created a page inside GitLab where you can check commonly used html and css elements.
When you run GitLab instance locally - just visit http://localhost:3000/help/ui page to see UI examples you can use during GitLab development.
All design files are stored in the gitlab-design repository and maintained by GitLab UX designers.
We are in the process of capturing our UI paradigms on brand.ai, see https://brand.ai/git-lab/primary-brand