Can Someone Explain The Css & HTML for tumblr?

Yes I understand theirs a page that define the blocks, what I don’t understand is how to customize it, I guess the blocks telling me the divs right?

.post
.link
.video
.quote
.photo

But how can I style them? I looked around alot of themes and I don’t understand, and I’m wondering if someone who does understand tumblr, make a simple tutorial of the tumblr blocks, or atleast help me out.