When you look up antonyms, you should make sure how to use them. Just learning vocabularies without knowing how to use it is counterproductive, as you might learn a wrong meaning by equating the German translation to the English meaning. I would suggest to check up the exact meaning of the word in a dictionary (e.g. duden, there are many examples). When you are still not sure, write some sentences and paste it in the notebook section for corrections. Licht is for example not the opposite of dunkel. The opposite would be 'hell'.