With prefers-color-scheme, web developers can provide styles specifically for dark or light mode. Recently, Firefox started to display dark mode specific styles to users who used a dark Firefox theme, even if they have their system set to light mode.

There is a flag in Firefox that determines whether dark/light preferences are taken from the browser or from the browser theme.

This is how you set it:

1. Go to about:config
2. Search layout.css.prefers-color-scheme.content-override
3. Set it to 0 to force dark mode, 1 to force light mode, 2 to set according to system’s colour setting or 3 to set according to browser theme colour

(via support.mozilla.org)

Personally, I feel respecting the system setting worked better. I hope it gets changed back. In the mean time, I hope this override helps.

Marcus Herrmann replied: prefers-reasonable-firefox-behaviour-regarding-dark-mode 👇

Marcus Herrmann replied: prefers-reasonable-firefox-behaviour-regarding-dark-mode 👇

dickelippe @ home replied: Thank you for sharing! I was kind of annoyed about that behavior for quite some time as I use both the FF Standard Edition (light theme as default) and the FF Dev Edition (dark theme) to develop and test websites...