A western theme party calls for cowboy related fun cake designs.
You could create a western scene, with cowboy figures placed on top.
For a western cake you could also do a drawing of a cowboy, similar to this cowgirl cake. (Making a "wanted poster" of the cowboy would be cute too!)
Here are some more ideas:
You can also make a cake about cowboy related items, such as a horse cake or a star shaped cake to be a sheriff's badge.
Show us your great birthday cake and tell us how you did it!