The question is, how wide you want it?
Right now it's 450px wide, Do you want it with a width of 500px at least?
I'd strongly recommend you not to tweak your CSS that much if you have little or no knowledge of CSS because tweaking one element, might lead you to tweak another so that it looks well with your first tweak and so on... and if you don't know what you're doing, you might end up with a messed up blog. Just a recommendation.
If you want your postings area to be at least 500px wide, then modify the .narrowcolumn selector like this:
.narrowcolumn {
float: left;
padding: 0 0 20px 20px;
margin: 0px 0 0;
width: 500px;
overflow: hidden;
}
If you want it wider than that, is very likely that you'll have to modify other elements in order to accommodate the new width of your posts area, so... I'd recommend you to take that into consideration before you do it or choose another theme.
To change the navy blue bg, add the following to your CSS:
body{background: [color you want] !important;}
HTH