When I exclude the sidebars from a specific page, the center column automatically adjusts its width.

How do I keep it fixed?

You could add a HTML/CSS Inserts -> CSS Insert

.coltwo { width: XXXpx; }

XXX = width in pixels

It worked! Thank you.

I added that one line in the style.css file.

Can I add a separate CSS file like custommod.css that will override style.css? This way, everytime there is a change in style.css whenever I upgrade, there will be minimal modification.

Just wondering. Thanks again.

Sure, you can add as many CSS files as you wish. You'd have to reference each additional css.file in header.php (just like you'd do with any web site)

PHP Code:

<link rel="stylesheet" href="<?php echo get_bloginfo('template_directory'); ?>/your/css/file.css" type="text/css" />

<?php echo get_bloginfo('template_directory'); ?> creates http://www.yoursite.com/wp-content/themes/atahualpa