qBittorrent official forums

is there any dark mode? If there's any then can anyone tell me how to activate it?
using qbittorrent 4.1.7 and Windows 10 1903

no, there is no dark mode (neither any color customization options) in the official build. If you wish to learn more about unofficial dark mode development u may wish to have a look there
https://www.reddit.com/r/qBittorrent/co ... lable_now/
be warned most probably you might end up with a torrent client reinstall but not a working dark mode

I really like the dark theme but is there anyway to convert it to work with qBittorrent Portable from PortableApps?

Yes, upgrade to v4.2.2 which has a fix for themes in it and then use the ayuDark.qbtheme theme from this url.
https://github.com/maboroshin/qBittorrentDarktheme