Do you happen to have a link to the HTML theme you want to convert?
My suggestions would be to:
1. Study the Dev docs: https://getpublii.com/dev/
2. Download a free Publii theme that resembles the one you want to use the most, and study it. Go through allll the files and folders in the input > theme folder, and do your best to understand how everything works.
3. If you haven’t already: learn HTML and CSS. freecodecamp, mimo, and Plan Design Code are good places to start.
Good luck!