How can I change the main CSS codes?

[merefield (Robert)]

You are best to start here:

Beginner's guide to developing Discourse Themes

So, you want to create Discourse themes? Not sure where to start? Or maybe you have created Discourse themes before, but want to learn how to do even more cool things. Well, you’ve come to the right place Developer’s guide to Discourse Themes Subjects include a general overview of Discourse themes, creating and sharing Discourse themes, theme development examples, searching for and finding information / examples in the Discourse repository, and best practices. Prerequisites: …

and it will not slow your site down.