Well, thanks to Ghyath Serhal, I found that I cannot create a 'button' style like this:
$("#nextday_button").button().css('font-size','36px');
because this code only puts a check in the box. I should stick a label instead, so I assigned the identifier to the label, and then changed JS to:
$("#nextday_button").button();
$("#nextday_label").css('font-size','36px');
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