Themes: In the reader pane I would like to be able to change the header color

With conversations enabled the reader pane may can many msgs. Currently when these are collapsed they are rendered with a light grey background, when they are expanded they use white and msgs are seperated by a light grey line.
This makes it very hard to see the message boundaries when you have expanded a bunch of messages in a conversation - there are grey lines all over the place from the nested msgs.
I would like to either change the header background color so it is obvious both when collapsed and expanded, or change the color of the seperating line to something more distinctive like red.

Using V7 I hacked the attribute - to gave a red dividing line (it changed a few other things as well but I didn’t care) Under V8 this isn’t currently working.

Is there a better solution or an attribute for the headers?

appears to control the header color when they are collapsed, but I can’t find what is used when they are expanded

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