I looked at a lof of b2evolution skins and it looks like most of them use fixed widths. On wide monitors this leaves much of the screen unused. Is there an easy way to modify the nifty_corners skin so that the main blog column expands to fill any remaining space so that the whole screen is used? This would leave the other column (the one with the calendar) having the same width that it has now. Keep in mind that I have never used b2evolution before and have barely ever used style sheets.. Be gentle
Basically you'll have to learn CSS to make a skin do what you want. Actually, start using the software and look at the skin's _main.php file and associated CSS file when thinking about specific aspects you want to customize. As you look through _main.php you'll be able to see that a particular block of code makes a particular piece of the page, and you'll see where styles are called out for each specific bit of the block. So then you open the stylesheet and find what each particular bit is defined as.
I like http://www.w3.org/TR/REC-CSS2/propidx.html when I need to look up how a particular css thing works or what I can do with it.
In that particular skin you'd be interested in the overall width of the page. "<div class="wrapper">" owns that, so looking for "wrapper" in the stylesheet shows me it says "width: 105ex;". Next you would notice that the main body is defined by "<div class="posts">" and that "posts" gets "width: 75ex;" in the stylesheet. Finally the sidebar comes from "<div class="bSideBar">" which has "width: 75ex;" in the stylesheet.
With me so far? Good! Now you tinker. I'm pretty sure using ex as the width means "the width of a letter x", which is why that skin gets proportionally wider if you increase your font size. That'd be no good if you set it to really big numbers and a visitor had a smaller monitor and larger font than you, so you'd probably want to change them to percentages. Maybe the wrapper is 90% (of the full monitor width), the posts gets 68% (of the wrapper) and the bSideBar gets 29% (of the wrapper) would be nice. 68 and 29 don't add up to 100 because you'll always lose some room due to how different browsers interpret the box model differently, so you have to tinker and look at it in a few browsers at a few different resolutions to be sure your visitor doesn't see something really ugly. It's okay that they go over 90 though because you're talking about percentage of the containing block level element, or the div wrapper you set to 90% of the screen.
So anyway that's an extremely complex skin to start learning this stuff on, but hopefully you see why my first line here is that you would have to learn css to customize a skin.