I've read that kdenlive will use the gtk theme that is applied but for some reason everything is staying light for me. I applied the default adwaita dark theme with the toggle button in the tweak tool. I have not tried other themes because I don't like how the close/minimize buttons end up looking (at least on the few themes I've tried).
Somewhat related: When using a dark theme, window borders on non gtk apps are not dark. Is this expected? If so, why?
The KDE theme settings stated above will not help because that is for the KDE desktop environment and not the program “Kdenlive” (If you are running gnome/ubuntu standard, mint, etc you don’t have KDE) In order to get all the extras for Kdenlive including the dark theme run the following command after Kdenlive is already installed.
sudo apt install kde-runtime
There you go let me know if you have any trouble. I had a hard time finding this out as well.