It’s tricky. You can use the about:debugging page to open the Inspector for an extension, disable auto-close of pop-ups, open the popup, switch the focus of the Inspector and then use the Rules panel. Debugging | Firefox Extension Workshop
It might be simpler to modify the global preference.
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
(2) In the search box in the page, type or paste the new preference name
ui.systemUsesDarkTheme
(3) On the bar with the bolded preference name, click the radio button next to Number and click the + button to add the preference. Then Firefox should display the new value.
(4) Set the value as desired (double-click to edit if necessary) and click the blue checkmark button to save the change.
- 0 => Light
- 1 => Dark
- 2 => No Preference