Also when ! appears and looking at their dialogue i have no clue what to do to make them happy? Put up paintings or lava lamps maybe?
It depends on what they're complaining about exactly. If it's the surroundings then maybe there's trash around? Better furniture does indeed also improve their work quality. You can check the specific furniture bonuses when you expand the item in the furniture buying popup.
If the loyalty of the employee is low then they'd like a pay raise. Click the edit button next to their wage to change it.