I fixed the post_form.htm, the code should be as follows(for JForum 2.3.5):
<select onmouseover="helpline('s')" onchange="bbfontstyle('[color=' + this.form.addbbcode30.options[this.form.addbbcode30.selectedIndex].value + ']', '[/color]')" name="addbbcode30">
<select onmouseover="helpline('f')" onchange="bbfontstyle('[size=' + this.form.addbbcode32.options[this.form.addbbcode32.selectedIndex].value + ']', '[/size]')" name="addbbcode32">
now the color and font should work as expected