I've had a poke around in the TweetMeme plugin code and IANA JS Guru, but I cant find any sensible way to make specific styling changes; you can add div-specific CSS characteristics, but I'm talking about changing the resting colour and the hover colour of the button.
I've googled around and noone seems to have asked it on here so I thought I should.
The button uses an image sprite for its background and hover states. But this is beside the point because the button is loaded from the TweetMeme server in an iframe, which makes overriding the styles much, much more complicated (if possible at all). If it were possible, you'd need to alter the content of the iframe with javascript after everything has loaded. You can't just edit the plugin to make it serve what you want.
I suggest finding a plugin that exposes its elements to CSS styling if you need to have control over its look and feel. Since there are many such plugins, that probably won't be too hard.
You could always wait for their Pro version of the button. I hear it's due out in the not too distant future.