Well, you would already have some themes for your vim, by default. And if you did not like them then you would have added some of your own too (downloaded from vim.org). But, those are something, that you might not still like and want to make some changes.
Here’s, a new way to do it. Just go to the link mentioned below and click on Generate Dark or Generate Light. You can generate as many as you like and once you like the theme, simply click on vim in the bottom and download 🙂 BTW, you can generate textmate and Emacs theme as well.