There isn't anything specific... just whatever fits the theme of that particular round best. The theme list is as follows:

Autumn
Business
Childhood
Color
Communication
Competition
Danger
Education
Emotion
Entertainment
Finance
Food
Fun
Health
Help
Love
New Year's Eve
Peace
Power
Relaxation
Security
Shopping
Spring
Stress
Success
Summer
Teamwork
Texture
Transportation
Unique
Water
Winter

Of course, we're open to adding more themes at any time. Those are just what we came up with!