Can't edit css file to get a bigger header logo

Hi, folks.
I’m posting this because, despite having tried various hints published on this forum, I’m still facing the same problem: I chose Busby theme for my blog and now I apparently can’t set the top banner to occupy a bigger share of the header. I don’t think I have to point out that I’m not an experienced user or whatsoever, but I tried to follow diffent advices up to know. This is my blog, hope someone can help.

Thanks a LOT!