Headcannon time
One time Yosano brought some nail polish to work cause she was bored, and Kenji saw and asked if she could do his,and then he showed Kyouka and she also thought it was cool. So now they paint their nails together whenever they all have a little free time during work. (Kenji really likes the sparkly nail polish that Yosano got for him, Kyouka likes hers to have little flower designs.)