One subsection per theme, one image per mode, stored in assets/ as .webp:
### Orikalk Lapis

How to install, then how to activate. Use the app's standard way of getting themes: its marketplace, store, plugin manager, or theme command. Only when the app has none and reads plain files, give one copy-paste command fetching from this repo, like:
curl -fsSL --create-dirs -o ~/.config/<app>/themes/"Orikalk Lapis.<ext>" \
"https://raw.githubusercontent.com/orikalk/<app>/master/themes/Orikalk%20Lapis/Orikalk%20Lapis.<ext>"End with the config line or menu path that selects the theme.
MIT, copyright Orikalk.