You will need to inspect the CSS on the browser and amend it, applying your customisations to a Theme or Theme Component.
This is the realm of Theme development.
Take a look at Beginner's guide to using Discourse Themes to help you with the Discourse specifics.
However, this is largely a general web development technique which is not the focus of this forum.
You might want to take a look at some guides like CSS Tutorial or CSS first steps - Learn web development | MDN