changing /?mystique=css stylesheets

Hi,

I have a problem with theme styles.
It contains some css settings, fixed sizes of the columns.
e.g.

body.fixed.col-3 #primary-content {
width: 500px;
left: 200px;
}

It is founded under http://www.domain.com/?mystique=css.

How can I change these settings? Because these are not founded in basic style.css.

Thx,
cavey85