It's <element id="Panel.ChildHeader.Default" bg="157,35,56" bg2="241,222,141" fg="244,238,195" bdr="240,202,86" />
The auto-generated lowlight fg colour works fine with only single bg, but when a gradient is applied on the header the lowlight doesn't always work well (refer to image in OP).
I thought the v3.4 element
<element id="ContrastBoost">0.2</element>
might be used to address this but no luck so far.