Some people have a theme to their Mishloach Manos. Is there any source that discusses Mishloach Manos themes, either pro or con?
Not sure what you mean by pros and cons. But I can tell you that people these days take the whole mishloach manot "creativity competition" a bit too far and get stressed over it for nothing.
I wrote an article about some popular Mishloach Manot themes and ideas.
The "main" themes of Mishloach manot are catered towards people's likes. I for example love tea, so I would be happy to get a tea themed mishloach manot.
But don't feel obligated to stuff the mishloach manot with junk. For example, a classy "made in Israel" gift box with a bottle of wine and olive oil is a wonderful mishloach manot gift IMO (see the made in Israel section in the article above)