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Matching pairs and Japanese
There's an issue with the style in Duolingo where it asks you to match pairs. I tested this with the Japanese course by the way. The issue is that you can't tell if you already matched the pair or not because they're always the same color. Also the theme isn't the most compatible with Japanese because when they ask you what said hiragana for example (ki) the black of the letter is similar to the black of the background.