Give your ul an id,
<ul id='yourUlId' class="subforums" style="display: none; overflow-x: visible; overflow-y: visible; ">
then do
var yourUl = document.getElementById("yourUlId");
yourUl.style.display = yourUl.style.display === 'none' ? '' : 'none';
IF you’re using jQuery, this becomes:
var $yourUl = $("#yourUlId");
$yourUl.css("display", $yourUl.css("display") === 'none' ? '' : 'none');
Finally, you specifically said that you wanted to manipulate this css property, and not simply show or hide the underlying element. Nonetheless I’ll mention that with jQuery
$("#yourUlId").toggle();
will alternate between showing or hiding this element.