Well, we can put it in our todo list and wait until it's added to the script. Or we might introduce plugin system, similar to Instant Update's plugin system, so this and other features can be implemented using plugins...
Well, you already can change color of the speech bubble by modifying the CSS code, and every user can register account with Gravatar.com to get it's own avatar appear along with their comment.
Doesn't the speech bubble use grey graphics for the borders? so changing the css would just change the central part to a different colour but the edges would be grey.
Also no one is going to take the time to register at Gravatar, most people want minimum hassle and will only take part in something if it is extremely easy to do. Just adding a few fun features would make people more likely to take part, that is the only purpose of the comments script to get people engaged to the site, which will mean they are more likely to come back + the comments would help add content to my site, which is why I purchased the script.
Well, you asked how to change color of the speech bubble, not it's background or borders. To change background and/or borders, you need to edit images which form the background, and those are: comments/images/ctl.gif (top left corner) comments/images/comments_top.gif (top line) comments/images/ctr.gif (top right corner) comments/images/comments_bottom.gif (bottom left corner and bottom line) comments/images/cbr.gif (bottom right corner)
Also, soon we should release a template which will be much simpler for editing.
Gravatar is (as it name says) globally recognized avatar, which gives user possibility to have one avatar displayed on many web sites. Currently, our company policy is to use Gravatar for user pictures, and I doubt that will change soon.
Good thing about CSS3 is that you don't need to use annoying edge images anymore just for rounded corners. Just use -moz-border-radius and -webkit-border-radius, background colors, and borders and you're all set. Makes it super easy to customize. Maybe something to think about for the next version :)
The styles you have mentioned apply only for Webkit (Safari) and Gecko (Firefox) browsers. Until all browsers start to support css-based rounded corners we can't use these. We will probably make rounded corners using jQuery