Up at the top, click/tap settings, dark mode option is in drop down.

Since you mention permissions/access denied, have you given your user permissions to use serial? You can do this with:

sudo usermod -a -G dialout $USER

What is the output of:

ls -al /dev/serial/by-id/*

What hardware are you using for hosting klipper and mainsail? Are you using a pi and mainsail os or a pi with raspberry pi os and kiauh to setup klipper and mainsail? Could you post your klippy.log?