ADanishMuffin
Food is life~
Ehehe, some of you have so many themes! I wish I could afford that many, lol. 
Depends. I like seeing the themes, hearing the music and whatnot. I switch between games a lot so I end up on the 3DS menu a lot. Other people just play a single game all day so they barely see it. I think assessing how much time you spend on the menu gives a good idea of whether or not it'd be a good idea.I haven't bought one yet. Are they really worth the money?
On my main 3DS XL I've got the two free ones but haven't used them yet and I got the Halloween cat free one, I've purchased the ACNL Christmas and New Years ones and I'm currently using the New Years one. I'm thinking about getting the Onemuri Pikachu one to use as my main one.
On my other 3DS XL I'm using the Yoshi theme as it's the special edition Yoshi 3DS.
Just the defaults. This is the first I've heard about the free ones though so I'll be looking into that soon probably.