How The Hell Do People Change Their Site Code?

So, I've Been Lurking Around The Site And I Have Seen That People's Profiles And Dens Look Different From The Sites Default State. And I'm Just Wondering, "How The Hell--" Like I Wanna Do That Too, But I Have No Idea How.

The More I Delve On The Idea, The More I Feel Like I'm Missing Something Obvious Though. 

Hiya! It's CSS! There's some freebies on the "Layout & CSS Sales" channel and there should be some guides/help on the "HTML Help" channel!

Okay, Thank You