Having a theme for your party is not necessary, but it definitely helps. It helps you plan, it makes decorating easier and gives you something work with. It is my favorite part of the party process!
Children's party themes are relatively easy to come up with. What is your child interested in? What movie or tv show are they obsessed with? If they are really little and it's a 1st birthday, what is their nursery decorated with? Or what year on the Chinese calendar were they born in? A recent inquiry had a Dog theme because their son was born in the Year of the Dog. There are all sorts of places you can look to for inspiration.
If you are planning a baby shower, ask the Mom to Be if she has nursery ideas and coordinate around that. If it is a gender reveal party, the options are limitless. Deer or Doe? Harry or Hermione? Wendy or Peter Pan? You get the idea.
Even for an older person's birthday party you can have fun with themes. Keep it simple by focusing on one color and use various shades. Or go with a classic Gold and Black. You can also jump into the deep end and work around their favorite hobbies or interests. Have a Mermaid party for your Mom who is turning 50. Why not?? Just have fun. :)