WP theme edit

Hi,

I've started to build my new website.
I bought a hosting and a domain, and I installed WP (using free theme).
Now I want to modify that free theme.

I have some basic knowledge on CSS and HTML, so I did about 80% of modification.
But, now I'm stuck with some silly problems.

I can't change navigation menu (font, size, color) and I can't modify boxes on the right sidebar (borders, background, etc.).

This is a screenshot of my webpage:

And my CSS file:

Any advice?

Thanks in advance,
Marin