Best case scenario: you decide your theme first and then start choosing the venue. Why?
The reason is simple: each theme has a set of colors associated with it. A jungle theme would be green, yellow and brown, Hollywood equals black and silver, underwater world means shades of blue and some greens. The colors of the walls, carpet and even ceiling is the backdrop of your decorations and they have to match, or at least not conflict. This can go up to the point that sometimes you end up change the theme or looking for another venue for this very reason. For example: we've decorated a wedding. It took place in a bright white tent and everything was white: chair covers, table cloths, bride, brides maids, groom and best man. No prizes for guessing what color balloons they wanted. How are you going to find the party when everything is the exact same color? And so on and so forth.
The second biggest thing is the height of the ceiling. Unless you have a very small room for a party of, say, 30 people, the taller the better. It gives us a lot of freedom in choosing decorations. If ceiling is high enough, one big balloon sculpture can be used to bring out the theme beautifully.
What exactly is a big sculpture? It depends. Once, we had a huge valentine party (1200 people!) and not nearly enough of budget to dress it up completely. So, we made two big red hearts floor to the ceiling and placed them to the right and to the left of the stage. We did not have a cent left for anything else, but the room looked very good. That is a major benefit of big balloon sculptures.
Once you took care of the room at large and of the first impression ("WOW!"--the name of our company) you can concentrate on the tables (centerpieces), the dance floor, the cake table and so on. This is the main reason for this article: be careful to how far you push the idea of the theme. In some cases it comes easy and natural: if it is just black and white party, you use black and white everything: balloons, flowers, plates, napkins, table chairs and so on, and the same with a Spiderman party, big sculpture in the middle or on the wall against the entrance, and then use the same red and black colors everywhere.
But if the theme is, for example, the name of the birthday boy in silver, then the deeper you go the more you spend and the less you get: printing name on napkins and forks, on placement cards and centerpiece containers is all a special orders and is expensive, but being smaller and smaller gets very little attention.
So there, as usual, the main thing is a balance between the budget the client has, the size of the room and the theme.