Can't change class

I’m using a theme called Gazette, and I’m trying to change a columns width based on which category that’s being displayed at the moment (column 1), by using the following code in index.php:
<div class="col1" <?php if (in_category('3')) {echo "style='width:640px;'";} else {echo "style='width:585px'";}?>>

It might not be a perfect solution, but that’s not the funny thing… The funny thing is that no matter what I do, nothing seems to change. I even tried removing the whole <div> in index.php, but the site looks exactly the same. It’s like it’s only there (in the code) for show, and that the theme creators doesn’t want you to change it.

Ideas, anyone?