Tonnes of great themes work well for both boys and girls, and all can be done in muted tones that don't scream boy's room or girl's room:
To achieve this look in a gender-neutral way, use colours inspired by the jungle like browns, greens, khakis, creams, yellows with some punches of bright colour as accents. Try these understated and sophisticated wall decals that will help you complete your jungle theme without going totally wild.
Moon and stars
When considering a nursery theme for your boy or girl room, look to the heavens -- literally! A sky-inspired theme is a perfect way to decorate a both-sex bedroom. We can't get enough of this sophisticated yet soft moons-and-stars theme nursery from Pottery Barn Kids.
Designing your child's room like it's under the sea provides a great gender-neutral theme because the options for pattern, style and colour [pastels vs. bolder and brighter tones, for example] are truly endless. Of course, blue is a central colour here, but an ocean-themed room doesn't necessarily mean boy room. Incorporate fun colours such as pink, orange and yellow in the fish, coral and other sea life to give this theme a softer look. Check out Target's rendition of the underwater world for a shared nursery.
Choose a pattern, then use it in one colour for your son and another for your daughter. For example, instead of a theme, go with dots, stripes or a combination of both that can be designed in either a fun/funky or more subdued pattern.
Think small, similarly-sized dots for a more sophisticated room. Dots of varying larger sizes and colours can add some fun to a room.
Most simple nursery and childhood furniture can work with either a boys or girls room. Select a simple style devoid of ornate details in a simple wood finish that can complement any décor and multiple decors as the kids get older.
Older kids form definite opinions on how they want their rooms to look. Give them a little free reign to decorate -- but avoid the all-black look! Allow each child her own space within the shared room that she can decorate how they wish, and give them a little freedom of expression, such as choosing their own pillowcases or cushions. But keep the main overall theme, along with bedding and furniture, coordinated to keep the room tied together.