This is a very old question, but in my case relevant and as part of the original question wasn't answered, I too would like to know how to modify the sidebar / content widths.
I've tried custom.css but this doesn't seem to work.
@media (min-width: 1200px)
.sidebar {
width: 20%;
}
@media (min-width: 1200px)
.one-sidebar #content {
width: 80%;
}