The default UI theme should consult the web browser to determine if it should default to light mode or dark mode, such as by using the prefers-color-scheme CSS.
The LuCI interface already does this, which can be verified by right-clicking on the LuCI page and clicking on 'View Page Source' on Firefox.
Using the browser/system prefers-color-scheme setting for the Default theme would provide a more consistent experience when switching between light and dark modes.
We have recorded this request and will forward it to our development team for further evaluation.
Thank you for helping us improve the Web Admin Panel experience.