Hello there! Our tumblr layout is actually a modified version of our other blog, @nochocolate. The layout of nochocolate was commissioned from Tumblr layout designer extraordinaire, @estersand. All credit goes to her for the layout genius.
However, that was a long, LONG time ago and I'm not sure if Estersand is still taking commissions. I'm also not sure if custom themes on tumblr work the way they used to, since tumblr (seems?) to be moving away from custom themes and to a more standardized mobile-friendly format.
All that said, you can still access custom blog designs by going to your own blog's main page, clicking "edit appearance" (it's an icon on the top right that looks like an paint palette) and from there you'll find an option on the left panel that says "browse themes". There are a bunch of premade themes you can use and modify using the controls in that left panel. No HTML savvy required.
Just note that Tumblr HTML is kind of delicate, so save often and try not to make too many changes all at once.
Best of luck with your blog and story!