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That code above won't work due to the css classes are wrong but it would go into your css file under style manager.
Spoiler code generates
<fieldset class="spoiler" onmouseover="this.lastChild.style.display = 'block';" onmouseout="this.lastChild.style.display='none'"><legend><b>Spoiler</b> <small>(hover to show)</small></legend><div class="spoilerbody" style="display: none">test</div></fieldset>

You would need to override the javascript and do onclick instead of onmouseover
Could be done with jquery it looks like but I am not much of a javacript coder.