There seem to be two schools of thought on designing spaces for very young children. The first is to create subdued spaces with very little color. I call these vanilla rooms or lullaby rooms. They are calming, sweet, and make us all want to curl up and nap.
The other approach is stimulating with color and activity. These rooms are fully of fantasy, whimsy, and playfulness.
Which is best? Neither. Both.
It truly depends on the parenting style. Will your child sleep in this space, or is it a hub for changing, clothing, and playing? My own children co-sleep, so our best solution was to create a play room that was full of imagination, a mural on the wall with birch trees give our sons the option to imagine themselves outdoors. Illustrations from children's books adorn the walls, and toys, toys, toys... too many toys. At the same time, we kept it simple and unassuming, avoiding a theme. We did not want to dictate what our children imagined, just give them the tools to inspire and motivate them!
If your child is using the space mainly for sleeping, by all means, keep it relaxing. As a mother and a designer, the saddest thing I encounter is a beautiful space for a child that has forgotten the wonder of imagination. Keep it simple and subdued, but give them something to look at, and remember that in those first few months your baby cannot see color well- so red, blue, black and white are fantastic ways to stimulate their budding imaginations and curiosity.
When designing your nursery, the first rule should be function. A place to sit, comfort, and feed your child is a necessity. Make sure that you are comfortable in the seat and that you'll be comfortable in it as you nod off from time to time (I cannot recall how many times I've nodded off myself in trying to soothe my boys to sleep). Keep changing needs close to the changing area- you'll need one hand on baby while you grab for that extra diaper because s/he went again in mid-change! Function, function, function- then, of course, imagination.
As with any well-designed space, a theme may come through as less of a theme and more of an abstract concept. However, this is a child's room, so break from expectations and let your own imagination soar! If your child is old enough, ask him or her what they want, and have fun. Maybe he wants to live in a space ship with tigers. Maybe she wants to live in a jungle castle. Imagination is so fun! If possible, step back from the literal and abstract their ideas.
For example, the tiger spaceship may have grey walls and modern furnishings, a rocket nightlight and tiger striped pillows. We don't need portal windows and ship controls on the bed. I am a sucker for subtle murals, myself however. My own boys have Jupiter, Saturn, and clusters of stars on their walls!
Whatever you decide, function first. Then, let your imagination soar!