NOTE Using this method, you'll need to install xdotool for brightness keys
$ gsettings set org.gnome.settings-daemon.plugins.media-keys volume-down F9
$ gsettings set org.gnome.settings-daemon.plugins.media-keys volume-mute F8
$ gsettings set org.gnome.settings-daemon.plugins.media-keys volume-up F10
$ gsettings set org.gnome.settings-daemon.plugins.media-keys custom-keybindings "['/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom1/','/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom2/']"
$ gsettings set org.gnome.settings-daemon.plugins.media-keys.custom-keybinding:/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom1/ name 'BrightnessDown'
$ gsettings set org.gnome.settings-daemon.plugins.media-keys.custom-keybinding:/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom1/ binding 'F6'
$ gsettings set org.gnome.settings-daemon.plugins.media-keys.custom-keybinding:/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom1/ command 'xdotool key XF86MonBrightnessDown'
$ gsettings set org.gnome.settings-daemon.plugins.media-keys.custom-keybinding:/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom2/ name 'BrightnessUp'
$ gsettings set org.gnome.settings-daemon.plugins.media-keys.custom-keybinding:/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom2/ binding 'F7'
$ gsettings set org.gnome.settings-daemon.plugins.media-keys.custom-keybinding:/org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom2/ command 'xdotool key XF86MonBrightnessUp'