I found it myself in the end, changing those classes can change entirely the style of the AppBarCommand:
CSS class Description
win-command Styles the entire AppBarCommand.
win-commandicon Styles the icon box for the AppBarCommand.
win-commandimage Styles the image for the AppBarCommand.
win-commandring Styles the icon ring for the AppBarCommand.
win-label Styles the label for the AppBarCommand.
for example to square the circle:
.win-appbar .win-commandring
{
border-radius: 0;
}
.win-appbar:hover .win-commandring
{
border-radius: 0;
}
It's a hard job to ovverride all the default behaviour but it can be done.
Have a look at this for an example of the main styles: http://msdn.microsoft.com/en-us/library/windows/apps/jj839733.aspx