The setting of this series is supposed to be sometime in the indeterminate past.  But of course there IS no indeterminate past, so Seth had to decide on a time frame for the clothing and hairstyles of the characters.  He decided to take elements from the 70's, so that it would not be unusual to find someone with long sideburns or a large afro, but not so particularly the 70's that you see men in flowered shirts with wide collars.  
Without the word balloons, the coy images of Batman, as he negociates with Harvey Dent, have exactly the same character as the words do: the two men coyly giving each other just enough information to keep them on the project, but not let each other see their hand.