Sidebar colours, centring images

Hi,
I’m new to this and struggling to find some information. A friend set up my site and now I’m taking it over. I’m happy with the top and trying to make the sidebar look better.

How can I change the background colour on the sidebar?
I understand colours have codes like #222 or #f9f9f9 but I don’t know what numbers refer to what colours and I don’t know where in the css to find the number to change.

How can I make the images and other items in the sidebar be centred instead of lined up against the lefthand side?

Thanks,
David