Please help me with some basics

Hello everyone,

This is my first post. My major burning question is, what color pants can I wear with a really dark navy jacket? The jacket has two dark buttons. I feel that black pants look just fine and show some individuality, but some people hate mixing black and navy. Perhaps if the color and fabric are just shy of perfectly matching, it looks worse than a stark contrast.

The jacket fits very well, except that it is too loose around the stomach. I am thin, so should I get the waist of the jacket taken in? Would it flatter my slender body type? Or would it simply prevent me from wearing a sweater under the jacket?

I am also perplexed by a somewhat lighter navy sportcoat that I own. It has two big brass shank buttons. Can I wear black pants with that? How about considerably darker navy pants with a subtle pattern and a contrasting fabric? Any suggestions would be greatly appreciated, thank you.