i would say to start with css as you would with any other coding language and learn the syntax and structure of how it works using websites like w3schools and mdn docs. even if you wont use all of the properties available on jai due to the sanitizer, both of these sites are very useful for references and provide clear explanations and examples (mdn docs is much more technical so not as newbie friendly but i do prefer it because it also includes browser compatibility and experimental properties as well that are fun to read into.)

many of the resources i used for jai when i started specifically are unfortunately outdated but i would recommend this css carrd made by my friends rae and puppy since its most up to date. other jai specific resources:
- Puppy's JAI Page Breakdown (list of class ids/labels for elements on the profile page)
- Puppy's Guide to Media Queries (a helpful reference on how to make your profile design responsive on pc and mobile)

for just general usefulness:
- CanIUse (a site that allows you to search up browser compatibility of specific properties to make sure your profile works on different browsers)
- Flexbox Froggy (helps with understanding how to use flexbox (when you apply display: flex; to an element), i loved this game and it made me better at understanding and creating html layouts)
- Considering the User Perspective and Contrast Checker (both are helpful for making sure your profile is accessible and readable because what's the point of having a pretty profile if nobody can use it?)

those are the ones i can remember off the top of my head so i hope they end up being helpful to you 💖