Either choose a book from the Dr. Suess collection and work your theme around that or mix it up with fun elements from several books.
A fish bowl full of blue jello with plastic fish added makes a fun centerpiece. Fish shaped sugar cookies with bright icing. (like red fish, blue fish)
A collection of Dr. Suess books could be displayed, the adults will have fun picking these up and reminiscing about the ones they loved. A stuffed cat in a tall hat makes for some fun decor. Dr. Suess stories are known for their whimsical designs and bright colors that attract kids. You'll want to use lots of bright colors for this theme. A string of alphabet letters cut from bristol board and hung above the table would be cute too. Maybe some alphabet fridge magnets to add some more fun decor.
This theme allows you to really use your imagination here so just have fun and create some great ideas from the Dr. Suess' books.