Evenly display links across main menu?

I’m using the Toolbox theme, and I would like to have all of the links in my main menu bar be dispersed evenly across the width of the page, instead of being aligned to the left.

Currently, all that is modifying their position is the following code:
float: left;
position: relative;

Is there a CSS trick to get them to do this? I’ve been fooling around with auto widths and max widths, but this clearly isn’t working. Any advice would be much appreciated.