How do you change the background of the left-most mail account window?

Hi,

I want to change the background color of the left-most window/form that lists the email accounts, inboxes, local folders, etc. When I use the pick element option, it opens treeview under main form setting depending on where I click; however, this treeview setting does not have a ‘background’ option. I have tried changing the background to different colors under different settings, but they have not changed the left-most window/form. Is it possible to change the color of this section specifically?
I am using using the latest version (9.0.1708).

Thank you

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