De-emphasize refers to reducing the significance or importance of something. Its opposite, emphasize, implies giving more importance to something. Another antonym for de-emphasize is highlight, which means to focus attention on something. Emphasize and highlight both point towards stressing on something, whereas undermine is an antonym for de-emphasize in terms of undermining the importance or authority of something. Another antonym for de-emphasize is magnify, which implies enlarging or intensifying the significance of something. Highlighting, magnifying, and emphasizing, are all words that directly oppose de-emphasize by stressing on the importance of something.