Catching Elephant is a theme for Tumblr.
Please look through all the answers here before asking a question. The same questions keep coming up. A lot of customisation can be done with custom css. Try googling css background for example.Catching Elephant is a theme by Andy Taylor
You could make it wider. But you’d have to fix a whole load of other things in the process. Tumblr’s largest image size is 500px, unless you pull in the high res images (not recommended). So then you’re going to be stretching all images. Video’s also come in at 500px. The quote and link background images would need re-doing. That’s just the stuff I can think of right now.
Basically, it’ll break stuff for no real benefit.