I believe our theme was “getting married.” It worked for us!
Seriously, we started with ideas we liked, then looked for other ideas that worked with them. For example, we never picked our “colors.” Instead, we started with ivory dresses, so we wanted to have the chuppah (wedding canopy), kippot (those little hats Jewish guys wear), pew bows, and linens ivory, too. When we thought about what would go well with ivory, we both liked gold, so we added gold sashes to the chuppah and to the ceremony and reception chairs, and gold candle holders to the reception tables. We also reused the chuppah to frame the cake table at the reception.
We also found a ketubah (Jewish wedding contract) that we liked. The ketubah was displayed at both the ceremony and the reception. We got the permission of the artist to use elements of the design in our paper products. We used that for the invitations, programs, photo sharing cards, explanation of the Welsh love spoon we used as our cake topper, and thank-you cards.
So, we had elements that repeated throughout the ceremony and reception. However, we didn’t even try to make sure every last thing matched or fitted with an artificial “theme.” For example, all the chairs at the synagogue where we were married were blue. We could have covered them with ivory chair covers (which we already had for the reception). However, we decided that the blue looked just fine with our decorations, and left the chairs as is. The maid of honor already owned a burgundy dress she had used as Maid/Matron of Honor in another wedding, and we saw no reason to make her buy another dress. We used burgundy as the background for signs at the ceremony, and got pink flowers that went with her dress, but did not use burgundy at all at the reception (which was three days later, in a different city, and she did not attend). The dude of honor’s only suit was a navy blue, and his two ties were blue and red. We told him to wear the blue tie, so he would not clash with the Maid/Matron of Honor, but did not otherwise attempt to match him with anything.
Conversely, we used a lot of pinks and browns at the reception (brown backgrounds to signs; pink uplighting; amber, “natural,” and pink lanterns). They looked nice with the colors we were otherwise using, but weren’t picking up any colors in use at the ceremony.
I guess what I would say is that you should think about how the stuff you are getting will look together, as opposed to just looking at each thing in isolation. However, if it all works, you don’t need to drive yourself nuts looking for some cutesy “theme.”