How can I combine CSS sibling and child selectors/"combinators" properly?

Short question: If I want to pick the child h5 of a div with id "foo", only when the div is preceded by a sibling with class "bar", what selector combination should I use? Is this possible to do, purely in CSS?

I'm messing around a little bit in with some personal CSS on Wikipedia. I'm using the beta Vector skin (see < http://en.wikipedia.org/wiki/Main_Page?useskin=vec... > for an example) and I've added a few scripts (see < http://enwp.org/User:Nihiltres/vector.js >) that add menu items with extra actions. I'm self-taught in CSS/JS, so yes, some of that stuff might be very ugly. :p

One script, the one that toggles the left-hand sidebar, appears even on Special pages (pages generated by the software, e.g. Special:Prefixindex or Special:Watchlist, as opposed to normal wiki pages) and so it generates a menu when there might be no general actions that appear as tabs on the right. Because of the way in which the borders for the tabs are generated, the resulting menu top doesn't have a left-hand border. I know what property to set (I change the border-image on the h5 child of the div that constitutes the menu) but I don't know how to use the selectors properly for the given case. I want to select the child h5 of the #p-cactions div, but only if it (the #p-caction div) is preceded by a div with class "emptyPortlet".

I initially tried

.emptyportlet + #p-cactions > h5 {…}

but that didn't seem to work properly (which seems obvious in retrospect: it's looking for the h5 having the sibling), so I'm asking here. Any ideas?