Where do I find this very simple Tumblr theme?

I know this is a very irrelevant question and quite random! But I am wondering if there’s any Tumblr users here that are willing to help me. I’m looking for a commonly used Tumblr theme which all these Tumblr blogs seem to be using:

(I can only post two links due to being a ‘new user’)

Any idea where I can find them? Many thanks for any help!