CompletionWindow.Background and CompletionWindow.Foreground don't do anything.
So... I wanted to make the CompletionWindow dark, so it fits my dark theme, but the CompletionWindow.Background and CompletionWindow.Foreground properties don't do anything.
I'm not sure if that's intentional, but if so, why do these properties even exists?
After a quick look over the code, I think you would have to set the Background/Foreground properties on CompletionWindow.CompletionList, but I guess that won't work either because it seems template bindings are missing on the control template - so this is a bug.
FYI, CompletionWindow.Foreground and CompletionWindow.Background are there because of inheritance (CompletionWindow derives from Window). They don't have any effect because the ListBox within the template of CompletionList is using its default style and that is not transparent (AFAIK). Plus, I don't think these properties inherit their values, they are only valid for the window but not its children.
I searched some hours today how to change the Foreground of CompletionWindow because of my white text on white background issue. I'm looking forward for a qick merge of bugfix in near future. Thx