Thanks, Leo.
I had been using this code, which seems to work sufficiently to expand the entire menu structure. What I was hoping for is that only a top-level menu section where a submenu has been selected would be open, and all other top-level sections would remain closed.
.slideout-navigation .inside-navigation {
padding: 0;
}
#generate-slideout-menu .sub-menu > li > a {
padding-left: 30px;
}
#generate-slideout-menu .sub-menu .sub-menu > li > a {
padding-left: 60px;
}