I selected the built-in github page theme earlier for the github.io page, but ended up using my own html / css file. However, as a beginner webdev, I couldn't find a way to delete the theme, even after deleting the .yml file. Any advice? Thank.
Jekyll themes are configured in _config.yml .
_config.yml
Open _config.yml , delete theme: minima or the theme you have chosen and create your own structure.
theme: minima