first is your theme more general?
is it a color combination?
is it based off of something like a book, movie, character/celebrity, or an era in time?
if your theme is more general are there elements and specific colors that are associated with it? patterns? types of food?
for example a party with a Jungle theme could use the color green for table clothes, and mix animal prints like tiger, leopard, and zebra prints for paper goods like napkins, cups, and plates. and for food you could do a lot of different things inspired by jungle animals, you could do tropical fruit, meat kabobs, different desserts that use coconut, pineapple, and other fruits.
the more specific you can be the easier it will make things. also are their ways you can personalize it ( after all it is your party, make it about you!)
if it's a color combination why that combo, are there things you associate with that combo that are the reason that you chose it.
try to ask yourself these questions once you have picked out a theme, and it will make it alot easier for the next step in party planning the decorations!