Change GTK+3 icon theme for a specific application

Firefox doesn't play well with dark system themes and I want to use it and only it with a light system theme, which can be done with an environment variable, but then it will still use my default icons for the menus, which are also light and turn almost invisible.

How to open a single GTK+3 program with a different icon theme? I've only found a solution for GTK+2.

Solution 1:

How about:

mkdir -p /home/$USER/.firefoxconfig
export XDG_CONFIG_HOME=/home/$USER/.firefoxconfig
lxappearance #or similar to select theme and icons

and then start firefox as:

XDG_CONFIG_HOME=/home/$USER/.firefoxconfig firefox

As far as I know, firefox itself does not use the folder ~/.config. If it does (or some other application for which you want to do the same as above), you might want to symlink relevant files/folders to .firefoxconfig.