od rsjtdrjgfuzkfg (Vývojář) v Srp. 27, 2011 · trvalý odkaz

From the technical side of view that shouldn't be a problem, but this theme is ment to stay compartible to all aero colors, so a blue background would go against the concept of this theme. Imagine you have red or green as aero color (so the transparent areas are tinted red / green instead of default blue), a blue background would ruin the design. But as this theme serves all modifications for the look you want except the blue tint, you could do the following:
Copy and paste the following into a file with the name "userChrome.css" (you have to display file extensions in Explorer to rename the extension) in a subfolder "chrome" of profile directory (see http://support.mozillamessaging.com/en-US/kb/profiles#w_how-to-find-your-profile for details how to find it):
.toolbox-top{background-image: -moz-linear-gradient(top, rgb(235,235,245), #EEE, rgb(230,230,230)) !important;}

I don't have a thunderbird to test here right now, feel free to send me an eMail if it doesn't work, I'll try to find another solution for you then.