Settings has a screen mode under display. It's that what you mean? Where is this banding you speak of? Settings background is black.
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Now that you mention it, I only really noticed it on the initial setup screen after I installed the ROM. Any of the ICS-style backgrounds (which that initial setup screen uses) have nasty banding on them, but you're right in that I guess it's not really an issue since the menu backgrounds in TouchWiz are black.
The setting I'm talking about is different than the one you mention. There are two mDNIe options - I think one's called "mDNIe scenario" and the other is "mDNIe screen mode" (or something like that). The setting you're talking about is the latter, which has options like dynamic, standard, etc. The mDNIe scenario, OTOH, is something different.
I guess it's kinda hard to describe, but anyone who's ever used AOKP or CM would know what I mean. The Galaxy S2 Settings app has two different mDNIe options you can change - the scenario one has a bunch of different options, and under AOSP ICS it's very easy to see that some of those options create noticeable banding on the menu backgrounds, while the default setting (called "Camera" like I mentioned before) gets rid of the banding almost entirely and the menu backgrounds become smooth gradients.
Anyway, to be honest I guess it's really not a big deal given the coloring in TW. I've been opening random apps and windows trying to see if I can find banding anywhere else, but haven't seen any. I'd still be curious if the Galaxy S2 Settings app would function under Samsung-based ROMs or not, as it'd be nice to have the option to change both mDNIe settings if needed.