Finding a theme for your wedding isn't crucial. Frankly, it might not even be a priority. But the truth is, a theme makes everything come together. A theme shows fluidity from ceremony to reception. When your guests leave your beautiful ceremony and arrive at your reception, they should feel like it is the celebration of the joyous event they just witnessed. They should not feel like they stopped at someone else's celebration.
Something I like to think about when creating a theme is using a common denominator between the first announcement of the wedding till the last memory the guests have of the reception.
Typically, I'll start with the invitation (or save the date card). Sometimes it is as simple as a symbol or a phrase. Taking the theme and incorporating it into the ceremony programs gives the feeling of connectivity from what your guests first saw to the first impressions of your beautiful day. Bringing that theme to life with decorations in your reception hall will help complete that fluid feeling. And if you really feel like going above and beyond, find a way to incorporate it into your thank you cards.
One of my favorite themes was a wedding I recently helped with (planning a wedding is the pride of a bride, I could never take that away from her). On the invitations the bride selected the verbiage " [Parents Names] ask you to join them in a celebration of love, friendship, laughter and family". Then I took that to the ceremony programs I hand crafted. And as a final touch we thought of and created head table decorations. Taking wooden letters, painting them the base color (bridesmaids dress color) and wrapping them with yarn in the accent colors (colors of the flowers). Love, Friendship, Laughter and Family danced through the heads of all the guests of the wedding.