Themes Vs. Mechanics
Werewolf
Werewolf is a fantasy game, but a weakly themed one. Players aren't asked to speak in a particular dialect, or given werewolf masks to use. They are just told "there are werewolves, find them".
This supports the use of Werewolf as a social game. The theme is just strange enough to allow for humor in the game, which is a great way to break the ice. The theme isn't strong enough to scare people away - preventing the game from becoming playable by only werewolf fanatics.
The theme doesn't add to the mechanics of the game, but it doesn't detract, either. I've seen werewolf played with a Mafia theme. It could easily be skinned with any number of themes. Given this, I'd say the theme to Werewolf is actually not important, so long as there is a theme.
Set
Set is a great example of abstract theming. As such, there is very little to write about SETs use of theming. If I were to apply a theme to SET, I'd choose something that involves careful examination - such as examining virus. In SET: CDC edition, players are trying to track and cataloge dangerous viruses. A virus is composed of 3 bacteria. Bacteria can vary in shape, color, fill, and number of cells. A virus is created when 3 bacteria get together, and all the traits of the bacteria are all the same, or all different.
This theme works with the SET mechanic, since SET is about careful observation and grouping.
Cranium
Cranium, like Set, has no theme. But, it is a weakly themed, no themed game.
The art style of all the pieces of the game convey a feeling of light hearted competition. It's not a specific theme, but it definitely influences how people play the game.
It does mesh with the mechanics of the game very well. While Cranium is a progress game, it's not a serious progress game. All the activities a player may do are funny challenges - while some may be hard, nobody gets upset because they're also ridiculous.
Cranium is Trivial Pursuit for idiots. While this has nothing to do with the assignment, I'm going to point it out anyway.
Sissy Fight
Sissy Fight is a themed as a school yard fight between girls. It's not about punching each other behind the jungle gym, it's about teasing each other, and tattling to the teacher.
I've played a couple Sissy Fight games, and I found that most people don't actually talk during the game. When they do, it's usually to organize strategy. To me, this means that people who are playing Sissy Fight aren't getting hooked on the theme - nobody is talking in character, or making jokes about the situation. Sissy Fight is decidedly a non-serious game, but the people playing it seemed to be taking it a bit too seriously.
The mechanics of Sissy Fight detail a strategy game. That's where the game mechanics conflict with the game theme. The mechanics call for thinking about each move, analyzing possible counter-moves from your oponents, and then coming up with your own counter-counter moves. The theming of the game asks you to be an 8 year old girl during recess. The two don't necessarily mash up, which explains why people seemed to drop the theme when playing the game.
Casablanca
Casablanca puts you in either "The Resistance" or "The Occupation", in a war torn city. Without expressly saying so, the theme feels like you're supposed to be doing this during the 1940s. This is due to the very title of the game, as well as some of the areas you can visit in the game.
Players in the resistance are trying to build their resistance cells up, and players in the occupation are trying to infiltrate the resistance cells. Everything is accomplished through conversation.
The theme of the game does match the mechanics. When I was playing the game, I got in to the roll of pretending to be an ordinary citizen with something to hide. Because the game is meant to help people get to know each other, the focus on meeting each other as part of the game mechanic is greatly re-enforced.