My comment explicitly avoids the “standard” problem.
A user could have many "theming system"s installed at once, while only having 1 DE. The user ideally would configure only one, and some program should try to translate that system into the other ones.
Then each app will fetch the list of theming systems the user has installed, and choose whichever the app prefers. And if there’s no match, fall back to a default hard coded theme.
My comment explicitly avoids the “standard” problem.
A user could have many "theming system"s installed at once, while only having 1 DE. The user ideally would configure only one, and some program should try to translate that system into the other ones.
Then each app will fetch the list of theming systems the user has installed, and choose whichever the app prefers. And if there’s no match, fall back to a default hard coded theme.