As with most things, the Anodyne Discord server is a great resource for help with anything related to Nova. In addition to asking questions there, another great resource is the CodeIgniter documentation itself since that's the framework everything in Nova is built on top of.
Nova is built in PHP, a widely used server-side programming language that’s become increasingly fast and powerful over the years. PHP works well with HTML and databases, making it a great language for building dynamic web applications. If you want to tinker with Nova or even create your own extensions, learning PHP is crucial. These resources will get you on your way.
All of Nova's skins rely on a heavy dose of Cascading Style Sheets (CSS) to do all of the styling. If you want to change styling for a skin or create your own skin, learning how to write CSS is crucial. These resources will get you on your way.