I'm working on a new browser extension and I'm using Mozilla's web-ext tool to test it. I couldn't figure out how to run the test browser in dark mode.
So far I have tried this:
web-ext run --verbose --pref extensions.activeThemeID=firefox-compact-dark@mozilla.org
I figured out a way! Apparently, dark mode is an add-on; not a preference.
First step is to create an empty directory to store our Firefox profile.
mkdir ../dark-mode-profile
npx web-ext run -p ../dark-mode-profile --keep-profile-changes
Then enable dark mode and close Firefox (and hit Ctrl-C if necessary). Since we turned on the --keep-profile-changes flag, all the changes we made are now stored in our profile folder.
We can run Firefox with this profile:
npx web-ext run -p ../dark-mode-profile