gecko-dev/mailnews/ui/compose/resources/messengercompose.css

spring.spacer {
    width: 5px;
    height: 5px;
}

box.cleanborder {
    margin: 10px;
    padding: 10px;
    border: 2px groove white;
    width: 100%;
}

box.padded {
    margin: 10px;
    width: 100%;
}