Whatt is the best way to customize a theme?

Hi all,

I am fairly new to WP and web design and would appreciate some help.

I am running the 2012 theme.

My question is: What do I need to know in order to customize a them
the way I want it?

I know this is a broad question. My goal is to keep my site simple, however I want to be able to customize it the way I want it.

I am assuming I would need to know CSS?

For example, one of the things I want to do now is make the homepage a solid color. Currently it has a white border and want to get rid of that.

I appreciate all your input!

Thanks