php - Change Header Size of Boostrap Collapse Widget - Yii 2.0

Can anyone tell me how I can change the height of my collapse widget header in Yii 2.0. It's like 40px right now and I want to make it 20px. The container divs that the widget gets rendered with have class .panel and .panel-default and .panel-collapse, but I'm not sure if there is an option that I can set directly in the collapse widget or if I have to override the CSS somehow? Here is the basic code for the widget (the 'options' setting only works on the body content not the header).

echo Collapse::widget([
        'items' => [
                [
                        'label' => $mycollapseheader,
                        'content' => $contentstring,
                        'encode' => false,
                        //'options' => ['style' => 'background:black'],
                        //'contentOptions' => [],
                ],
        ]
]);