I would be shocked if something from Stripe was using an OTS theme.

It doesn't look like any off-the-shelf magazine theme. The source code is incredibly clean and not littered with organizational elements like Wordpress might emit.

It also most likely isn't an off-the-shelf magazine theme because there's no credit to any external author of the design in either the website or the code.

Also, the source for the page is in 1 single line. Doubt any off-the-shelf theme would do that!