Centering a form horizontally

This should not be giving me the trouble it has been giving, but I’m having trouble with it.

I just want to center this tiny form on the page.

Beyond that, I’m having trouble keeping the form looking good as I move to a narrow screen. I’d really appreciate some advice there as well.

I’m a developer, not a designer.

A huge thank you, in advance.

The page I need help with: [log in to see the link]