IMO that's a baseless claim by OP.
As it was pointed out by a a few posters above me, most casinos use dark or black backgrounds.
But let's see if OP is even a bit right at this.
The design of the casinos are very well thought around the psychology of people. The colour green generally indicates wealth, play and calm. Black is linked towards luxury and exclusivity. These colour compose make people think that they are in here for big money and premium vibes. Black UI supports less strains on eyes on longer sessions. Casinos pair them to make platforms feel authoritative, relaxing, and rich, perfect environments for keeping players engaged. It is never a coincidence. It is always designed to play with peoples emotions and control their behaviours to some extent (preferably as much as possible)