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
Changing the grey boxes is too hard because they all have images in the background. Changing the font size is simple custom css.
body {font-size:16px;}