Searchbar in K2

First of all - HTML/ CSS and I are bad friends, so this is probably an easy question:

I’ve got this sidebarless design in K2 (in the K2 options I’ve chosen “No sidebars”).
Therefore the searchcolumn is in the bottom, but now I want it to “fill it up” from left to right, and clear off that “X” on its right. How should I change this in the CSS?

For a better look at my site (to get the idea) see:

Thanks in advance!