Can I use Corolla theme as a fixed width layout?
Though I have not used that theme myself, it indicates on the project page that it's 'Responsive' so the sidebar is doing what its supposed to do when the browser is resized to a narrower width -- it's using a Media Query. I tried the demo and this was my experience as well. Though that might have seemed odd in the old days, now it's actually what you want for the site to work with multiple devices and screen widths. You will note that if you resize even narrower, the other sidebar goes underneath as well and this is as expected. 

From the project page: 


  "The new 7.x-2.x version is a subtheme of Adaptivetheme which includes
  a powerful new responsive layout engine with full support for mobile
  devices."


You may want to take a look at this for a primer on Responsive Design: http://www.alistapart.com/articles/responsive-web-design/