stvfrnzl, I love #presenter by @ia, but I think the documentation for creating custom themes could be improved. So I went ahead and did just that, hoping that it'll help someone to get started:
https://stevefrenzel.dev/posts/create-a-custom-theme-in-ia-presenter-reworked/
Disclaimer: Most of the words and all images in this article belong to iA, I rearranged the order of appearance and changed some words.