I would wait until you have a venue and let the venue guide your decoration. You will save so much money and stress if you don't try to shoehorn a look or a theme or decorating style into a venue, but instead work with it to just add any little touches that are needed.
Themes are hard if you don't yet have a venue. But choosing characteristics you want can help you look at venues, and say, how would we make that happen here. Romantic, soft fabrics, many shades of color , nothing hard or boldly contrasting. Or very Art Deco or Beaux-arts, a room with period furniture, very ornate. Or elegant and formal fit for a princess. Or sparkly as a swanky night spot, show biz touch. That can guide you to look for old mansions, or rooms and courtyards of a small museum, or a whole medium size restaurant room you would decorate as a night club. Getting a specific theme , I want a shipboard formal dinner, like a cruise ship of Titanic era; or a garden party with an English country house look; or modern and sleek as inside a space station; these are very limiting and will cause you to reject many places on looks alone. Are you most sociable in your yard, or at a day down by the lake? Do you look forward to very rare nights out clubbing? Like sitting around on couches or around small tables like a cafe? Choose one of this kind of setting choice, then a mood, and see what venues are out there.