There is too much platform-independent white background.
For one, the background on the outer part of the window is at line 25 of
<body style="background:white;" onunload="cleanUp();">
The background color should probably be "window". There may also need to be "color: windowtext".
The colors for the outer part of the tab and the pop-up windows are elsewhere.
We have dropped support for the Mozmill IDE in the upcoming Mozmill 2.0 release. Therefore it finally has been removed from the extension by bug 791625. Closing the bug as WONTFIX.