[netherking]

THIS IS A FIX, NOT A BUG REPORT OR FEATURE REQUEST

For quite a bit of time I've been unable to access the dark theme on my Ubuntu 18.04 PC, even with the fixes provided in the docs or on various forum posts. It occurred to my recently that there might be an issue where Ubuntu did not pass on the information that the dark theme was currently enabled to calibre. The fix was to set both environment variables to ensure it behaved the way I wanted.

To check if this works for you before implementing open a terminal and run this command:
export CALIBRE_USE_SYSTEM_THEME=0 && export CALIBRE_USE_DARK_PALETTE=1 && calibre

If your shell is bash (if you don't know what that means follow both sets of instructions):
go to your /home/USER/ folder
edit .bash_profile (create it if it does not exist)
add the following lines:
export CALIBRE_USE_SYSTEM_THEME=0
export CALIBRE_USE_DARK_PALETTE=1
save, logout, login and open calibre. Dark theme should be enabled.

If you use any other shell:
go to your /home/USER/ folder
edit .profile (create it if it does not exist)
add the following lines:
export CALIBRE_USE_SYSTEM_THEME=0
export CALIBRE_USE_DARK_PALETTE=1
save, logout, login and open calibre. Dark theme should be enabled.