Just wanted to add to the above:
Meta is not a dedicated support site for a complete guide to getting started with CSS — just like it is not a dedicated support site for learning Ruby and JavaScript completely from scratch, if you wanted to start customising the behaviour of Discourse through plugins.
If you want to customise the appearance of Discourse, you will need to learn more about CSS.
There are guides here on making themes, theme components and plugins which are specific to Discourse, but not for learning to be fluent in CSS or Ruby totally from scratch, since that’s a general skill. Sometimes people have time to kindly help, but it’s not always going to be possible, because it means having to go through general HTML & CSS knowledge – the basics of the Inspector and classes and properties etc.
This guide linked above can help you get started, as well as the Beginner’s guide to developing Discourse Themes:
If you would like to make extensive customisations — or maybe you just don’t have the time to learn as well — another option is also to post in the marketplace.