Not yet but it’ll happen. There’s a lot of work to be done with the CSS and there are some other issues too. The biggest problem so far is that it won’t display background images that use the data: URI scheme. I’ve filed a bug and hope it gets resolved soon. Another issue is that Thunderbird 3 is still in early development and I expect many, many changes until the final version.
Here’s a very early screenshot of AnyColor on Thunderbird 3.0a1pre.