make room for sidebar in single post layout

I’am trying to get a right sidebar into the single post layout, in the tesseract theme.
This theme does not allow sidebar on a post side as standard.

So far i managed to get the sidebar in the single post, but it appears in bottom,

https://www.airswitch.dk/2015/08/29/fablab-prototyping-facilities/

I do not now if this is a css or a php problem, as the content and entry-header fill the full width of the page.

thanks!