Why not adding a feature to skin Plus! completely? As it is exactly that what it is, "nothing more". Adding 1 option to change the layout of the sidebar menu is pointless...
Patchou and Guido have made the layout and look of Plus! as it is because they like it, it looks good, it is sufficient and has room enough for now.
Such an option will not be added just because a few like a more basic old plain look. There is no reason to change it (for the moment). And if there was a reason (lack of room in future updates), they would redesign it...
So, unless they add a complete skinning feature, don't count on such an option being added.
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