Mood come mostly with unique graphics and musics. A good mood is when we see a picture of a game and we can tell “Yeah it’s obvious, it’s from The Binding of Isaac” or “Oh look, Don’t Starve !”. You can also choose an idea, a feeling or an ambiance and make your game around it. You need to ask you : What i want the players feels ?! “Madness and the the seventieth spirit ?! Let’s make Hotline Miami” “A classic day at work : The Stanley Parable is perfect for that” !

You need to know exactly how the players have to receive the game. For your game especially it can be an epic quest or a marvellous adventure with wisps and dragons like Trine can do ! So maybe you need more variety in your colors and memorables monsters.

And for finishing, the mood can also be created by the narration.