Dark theme/Dark mode

[throtz]

I've been using qbittorrent for quite a while now and the white theme is killing me, i recently switched from windows (which has themes made already for u just have to "mount" it when selecting a custom theme in preferences) to Ubuntu 20.04, will upgrade soon.
On Ubuntu i noticed there aren't any themes and the white is killing me, so maybe look into a built in theme changer while still containing a custom theme option for the users that wish to have them custom?