My friend has a light green room, what theme could we decorate with?

you can always put up whatever kind of posters you like because color won't make them look bad. besides greens and blues (like the ocean) usually go together anyway. but if you want to play up the green than you could always try something natural and earthy or nature inspired. in the end its your friend's room and they should use whatever posters or decorations that make them the happiest because then no matter how it's decorated, the room will always seem happy to them. tell your friend to go with their instinct and choose what they really want, regardless of wall color. hope this helps and good luck!!