Also with RadListBox, of course. This is using the VisualStudio 2013 theme.
Create a ListBox with IsEnabled=false.
Add a CheckBox item, with a Content="Testing" or whatever.
Notice that it is very dark, nearly unreadable. It's because in your theme XAML, you have set the RadListBoxItem disabled state contentPresenter to an Opacity of 0.2. Now, inside of a CheckBox, you also have similar code for the disabled state where the content's Opacity is set to 0.2.
Guess what? The 2 values multiply, which means now the CheckBox has an effective Opacity value of 0.2*0.2 = 0.04. :( That's dark.
I can see why this was done, because if your ListBox has strings only, then it works, because a TextBlock doesn't have a disabled state. Yeah, but what if your ItemTemplate has controls that they themselves have a darkened disabled state??? That doubles up the disabled-ness.
I like how Microsoft handles it by changing the Foreground to grey in their themes instead of screwing with Opacity (since Foreground is inherited), and that would work perfectly in both my cases, string-only lists and checkbox lists.