There are only two themes at the moment: Fantasy and Modern. All scifi related backgrounds and effects (basically everything which lives in the bottom main category) comes in the Modern theme. Unfortunately there is only one Modern themed female package right now, which is the Modern Girl. It would unlock all the shared Modern content in your female characters.
There have been discussions before either to make this theme based restrictions or not, but there are quite a few positive sides of doing it (most notably having the Random functionality work nice), but yes, your argument is also valid, as if you don’t have the modern female pack (which would also come with a couple of cool modern backgrounds and items usable by others), you are not able to use your otherwise owned content in these packs. Hopefully adding another Modern themed female character in the future which could be useful for your characters can have this issue sorted.
Speaking of robes and weapons, it’s hard to tell whether these could be generalized or not, as the base characters are in different positions, they would require different rendering. We’re trying to have the same items available for both genders in different packs, but as we’ve moved away from creating gender pairs to a more sensible way of having different wardrobes, it takes more time to have the same items for all characters, because they come in different packages. Some of them are already there and available for all base characters, others will come in the future.