Centering an entire theme

Hey, first time poster, long time user!

I have a quick question that I know is just glaringly obvious (hopefully), but I still can’t find the answer:

How do you center an entire theme?

I’ve designed and coded to the point where I have this nice layout, but it is left-aligned. I have basically just hacked the default theme, taken out a few elements like the sidebar — and now I’m wanting to center it all.

Any suggestions?