"Do you want brown or white sugar in your tea?"
Translation:Möchtest du braunen oder weißen Zucker in deinen Tee?
My native language is German and I also wrote "... in deinem Tee" and I still think that you can use both accusative and dative here. If you use "... in deinen Tee" it refers to a direct act, i.e. the pouring of sugar into the tea. However, if you want to refer to the condition or state of the tea after you have poured in sugar, the dative (as a locative) will be correct.