You can find well maintained applications for almost everything in either qt or gtk3. So you can select one toolkit to support. KDE might even accept your theme if you did enough work on it to make it work - and this is something that is feasible for one person. The hard part is those few exceptions, if you need one GTK3 app because KDE doesn't have a good alternative (or one qt app because GTK3 doesn't) then you have twice as much work and it is probably beyond what one person can do. Making that missing app for qt/gtk3 probably needs a large team and a few years and so is beyond you (unless you are rich and can afford to hire someone to fulfill your vision) - and there are many missing applications, not to mention existing ones which are just barely acceptable