To make the popup menu in the left part of the start menu not have a separator or change the color of the separator you have to take the image and properties from Lists, Menus, & Tabs > Menu > PopUpSeparator and put them in Unknown > ImmersiveStart::Menu > PopUpSeparator and change the image color.
This is present only in the left side popups. Doesn't work on popups like Change Account Picture or Power Options. Can't find any way to edit those.
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