It’s nice to make an extensions Options background color in the addons page match the background color of the rest of the page.
In Firefox 88, I could use the following line of code to detect if the addons page was light or dark:
The color of the addons page only changed when the operating system theme was changed between light or dark or ui.systemUsesDarkTheme was set to 0 or 1 in about:config.
In Firefox 89, the about:addons page background color now changes to light or dark when ‘any’ Firefox theme is changed. If using the new ‘System theme’, I can check if it is in use as browser.themes.getCurrent() returns an object with all null values. If any other theme is chosen, though, I’m not sure how Firefox decides if the background should be light or dark.