Since the username is the same, I assume you are the same person behind
https://github.com/qbittorrent/qBittorrent/issues/13523 and
https://github.com/qbittorrent/qBittorrent/pull/13524. Please don't spam everywhere. Learn the rules of the sites you visit and lurk before you post so that stuff like
https://github.com/qbittorrent/qBittorrent/pull/13524 doesn't happen, and remember to search for existing issues/feature requests before posting a new one.
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Dark mode doesn't actually save energy unless a significant part of the theme is 100% black _and_ the screen in your device represents 100% black by turning pixels actually completely off. Typically, screens that do this will be OLED/AMOLED-type screens, which are much more common on modern smartphones than on laptops or desktops. On a regular LCD, surprisingly, a black background can actually consume more energy than a white background!
Side note: Black is also generally a better choice on non-OLEDs that have some form of dynamic brightness. For energy consumption, black is also a better choice for CRTs, but those are pretty rare nowadays.
All in all, I don't think implementing a dark mode in qBittorrent would lead to significant measurable energy savings.
That said, feel free to learn a bit and have a go at implementing it yourself! Good luck!