Change Background Color of Only Threaded Messages?

[ThunderStruck]

For Thunderbird 60+, in the threadTree view ("View > Sort By > Threaded"), is it possible to target and change only the background color of the threaded message rows? I can target and change the background color of the main container row which shows the thread's subject and which uses a twisty (-moz-tree-twisty) to expand its content—#threadTree treechildren:-moz-tree-row(container, open)—but I would like the background color of all of its child rows (the subject lines that are indented beneath) to be a different color when the container is opened/expanded. I can't determine the correct syntax to do this. Is this possible? If so, how do I do it?

[ThunderStruck]

Does no one know how to do this? I posted this over a month ago!