Very simple example of how you can use JavaScript within case themes.

case_theme.html

<div id="save_{{case.id}}">
<div class="agent_ticket_section">

{{subject}}

</div>


<div class="agent_ticket_section">

<div>

{{status}}

</div>

<div>

{{priority}}

</div>

</div>


<div class="agent_ticket_section">

<div>

{{group}}

</div>

<div>

{{user}}

</div>

</div>


<div class="agent_ticket_section">

{{description}}

</div>


<div class="agent_ticket_section">

{{language}}

</div>


<div class="agent_ticket_section">

{{tags}}

</div>

</div>
{{widget.jira.body}}

<button id="reset_{{case.id}}">Reset</button>
<script>
  $(function() {
    var temp = $('#save_{{case.id}}').html(),
        group = $('#save_{{case.id}}').find('#ticket_group_id').val(),
        user = $('#save_{{case.id}}').find('#ticket_user_id').val();
    $('#reset_{{case.id}}').click(function() {
      $('#save_{{case.id}}').html(temp);
      $('#save_{{case.id}}').find('#ticket_group_id').val(group);
      $('#save_{{case.id}}').find('#ticket_user_id').val(user);
    });
  });
</script>