Question

Why does

.right, .left .box_header {
margin: -10px -1.2% 10px -1.2%;
padding: 5px 0.5% 5px 0.5%;
}

not do what I would expect it to do (which is)

.right .box_header {
margin: -10px -1.2% 10px -1.2%;
padding: 5px 0.5% 5px 0.5%;
}

.left .box_header {
margin: -10px -1.2% 10px -1.2%;
padding: 5px 0.5% 5px 0.5%;
}

Answer 1

If you want to group several selectors, you have to fully declare them:

.right .box_header, .left .box_header  {
margin: -10px -1.2% 10px -1.2%;
padding: 5px 0.5% 5px 0.5%;
}

You cannot use , to group sub-patterns. See W3C CSS2 selectors recommendation or W3C CSS3 selectors recommendation.