I don't know of an exact theme, but I think you would be able to get a similar effect by using a standard landing page theme and tweaking the fonts / colors a little. Use a round typeface, double the font size, and choose vibrant colors.
Yup, I think just bumping up the font size alone makes a huge difference, along with increasing the line height and margins. I know some people dislike this trend but I quite like it. Panic uses a similar style for the Playdate website[1].