Having a theme is a good idea, it can be Disney, superhero etc. Choose what your kid loves the most. Does he/she like to watch Elsa from the movie Frozen? Then by all means use Frozen as your kid’s party theme.
The theme should show from your kid’s costume, to the invitations, to the venue and so on. Having a theme on their birthday would be really nice.
Note: When choosing a theme, it is highly recommended that you make him/her involved; it is his/her birthday anyway.