How to disable dark theme on webpages in Firefox?

The best solution to tell Firefox that it should use the light theme instead of the dark is by specifying the GTK_THEME variable prior to launching.

So, 4 ways you get for doing this..

1- from terminal you can do:

GTK_THEME=Adwaita:light firefox

2- if you hate the terminal and wanna close it once firefox works:

GTK_THEME=Adwaita:light firefox &

3- from firefox about:config you can set the following setting to false:

browser.display.use_system_colors

WON'T WORK FOR localhost SITES! so if you are a web developer/designer it won't work out that well for you.

4- if you want this to be the defualt behavior:

Just add two lines to the Firefox launcher script located at /usr/lib/firefox/firefox.sh

in the terminal type down:

sudo gedit /usr/lib/firefox/firefox.sh

look at the beginning of the script for this:

MOZ_LIBDIR=/usr/lib/firefox
MOZ_APP_LAUNCHER=`which $0`
MOZ_APP_NAME=firefox

export MOZ_APP_LAUNCHER

and change it to this:

MOZ_LIBDIR=/usr/lib/firefox
MOZ_APP_LAUNCHER=`which $0`
MOZ_APP_NAME=firefox
GTK_THEME=Adwaita:light

export MOZ_APP_LAUNCHER
export GTK_THEME

Finally, restart Firefox.

Hope this helps.

Please help me by voting this up.

Thank you :)

changing userContent.css solves problem ; more information is here also dolerean theme comes with the firefox tweak more information is here; just installing it is enough