How do you know if and what the theme nights are on your cruise?
I would ask your TA or the cruise line when you book. The only theme night I know of that is recurring is the "White Hot Party" on NCL.
It seems that there is always a theme night. Call the cruiseline head office to ask what is popular. On Princess they seem to have a sock hop. NCL white hot , and I know of a few friends who did a country night, I would like to take a cruise out of New Orleans for a Mardi Gras party...
A Mardi Gras night sounds fun!