Blaskan Theme, Sidebar to 300px, responsive

Hi Guys!
I’m using the Blaskan Theme for my site elephantasticvegan.com and I wanted to put ads (300px) in the sidebar, so I changed the width of the sidebar in the custom css.
Unfortunately, I destroyed the responsive design with that.
I would like to have the sidebar on the right and not below all the content when opening the site on mobile devices (ipad).
I’m the worst at media queries. Is there an easy fix?
I have the framework.css in front of me, but there are just too many widths I could change.
Any help is appreciated.
Thanks, Bianca