I liked your previous color choice more. Current button colors are to vibrant and thus stand out to much.
In order to make choosing of difficulty more intuitive I recommend you make use of the RadioGroup component. Ant to make radio group horizontal just make sure that you change its column property to the number of radio buttons that it contains (5 in your case).
You could still keep keyboard binding for choosing desired difficulty quickly and add two more with left and right keys increasing and decreasing the difficulty respectively.
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