How to fix FireFox's theming errors when using a dark desktop theme - Linux fix
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Today I show you how to fix a very common theming error which occurs when running Firefox web browser with dark Linux desktop themes. This should work on all desktop environments (e.g. Gnome, KDE, XFCE, Cinnamon, MATE) and on all Linux distributions (e.g. Ubuntu, Mint, Manjaro, Arch, Fedora).
Type in these commands into a command line terminal one line at a time, items in square-brackets are instructions and should not be typed into the command line terminal.
cd ~
cd .mozilla/firefox
ls
[Check for the directory for the profile you wish to fix]
cd [profile directory]
mkdir chrome
cd chrome
nano userContent.css
[Copy in this code: http://pastebin.com/My5byv4j ]
[Press Ctrl+X to save the changes and quit]
The source for this guide is here: https://wiki.archlinux.org/index.php/firefox#Unreadable_input_fields_with_dark_GTK.2B_themes