There are many Gmail themes to choose from, but few of them are actually usable. Gmail added 5 new themes: Basic Black, Basic White, Tree Tops, Marker and Android.
Even if you're not an Android fan or you don't have an Android phone, you should really try the Android theme. It's simple, stylish and you can still read your messages.
If you're looking for a minimalist theme, Basic White is a good alternative to Minimalist, an already existing theme. It looks a little bit unfinished, but Google says it's the Gmail interface stripped "to the bare bones".
What's your favorite Gmail theme?