While it is possible to select between "light", "dark" and "system theme" in the theme menu, it currently isn't possible to select the dark or light version of themes like Firefox Alpenglow.
For that reaseon, I suggest combining "light", "dark" and "system theme" into one theme called "Normal" or similar.
Instead there should be a new drop-down menu or similar which can be use to switch between the three modes. It looks like that there are useres who prefer to use the dark version of Firefox Alpenglow and a new theme which does not include the light version but always uses the dark version has been published. With such a selector, uploading a theme like that will become unnecessary.
This became especially important since the Private mode will automatically enable a dark theme. There should be two drop-down menus like that: One for private mode and one for normal mode.