Please describe precisely which element is not being styled in the rendered output of the component.

I've added my own style class to the collapsible panel:
<ice:panelCollapsible expanded="true" styleClass="myStyle">

css:
.myStyle

.myStyleHdr

The produced code looks like following:
div class="icePnlClpsblCnt">
<div class="icePnlClpsbl myStyle">
<div class="icePnlClpsblHdr myStyleHdr" ...>

The code seems to be ok, but my own style (background color) isn't applied on the header. The base css class (icePnlClpsblHdr) keeps the lead, so my class has no effect...

What browser and version(s) do you see this problem with? Seems like it could be a CSS issue.

The problem is visible only for nested components in IE 6 and IE 7. The style 'myStyle' (now with red background) isn't applied to the container (icePnlClsblCnt).
For Firefox it seems to be OK.

myStyleHdr does override icePnlClpsblHdr (its background color temporarily changed to red in rime.css), even in IE. See attached screenshot. The markup looks like this:

<ice:panelCollapsible expanded="true" >
<f:facet name="header">
......
</f:facet>
<style>
.myStyleHdr

</style>
<ice:panelCollapsible expanded="true" styleClass="myStyle">
<f:facet name="header">
......
</f:facet>
<ice:panelGroup >
......
</ice:panelGroup>
</ice:panelCollapsible>
</ice:panelCollapsible>

The result in IE looks exactly the same as in Firefox. So I don't see how it is not OK in IE but OK in Firefox.

Mark as resolved for now, until there is further clarification on what the true problem is and how to reproduce it.