HELP! I edited the CSS and now I can't even get to my dashboard

I’m still new to wordpress. I was trying to edit the theme functions.php file to make my tiled galleries fill the page. My CSS skills are not great. At first I added:

if ( ! isset( $content_width ) ) $content_width = 640;

and it was fine. But, using my stupid out dated skills I thought I’d try putting 100% instead of “640” there. And, now I can’t even get to my dashboard or see my site or anything. http://www.phoenixaviation.org/

We’re a non profit and we need our website. Please, help me!