If you are building or modifying your own css, you can open the developer tools by using
Ctrl+Shift+I on Windows/Linux or
Cmd+Opt+I on macOS to get information on the elements you want to customize.
Changes to themes and snippets will be detected automatically and reapplied. You do not need to restart the app.
New to CSS
If you're new to CSS, our community has put together a quick guide to get comfortable with CSS:
Klaas also put together a helpful index of common CSS snippets that you can feel free to mix and match: