Broadsheet aspect and logo

Hello everyone,
it s the first time that I write a post so forgive me if I do sth wrong.
I created my blog using Broadsheet, my idea was to put my logo at the top of the page. In fact Broadsheet allows you to do it but the shape of it is just a square. Because my logo has a rectangular shape it doesn’t fit in. So I think I need to use CSS. The only problem is that……… I don’t have any idea on how to do it (and I mean: It s the first time that I try to use a code :'( ). Can anyone help me, or at least suggest me how to start?

Thanks so much!

At the moment the webpage look like this:

https://www.thinktimeblog.com