As I am trying out Budgie, and needed to adjust my screen brightness via keyboard, but found that it ignored my keybindings from Cinnamon, I researched the issue and found the following works for me:
Open the Gnome settings app, go to keyboard shortcuts, then + to add one, call it Brightness up, and add the following command:
gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepUp
Then give it the custom keyboard shortcut.
For Brightness down do:
gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepDown