Idle? Member's name turns grey.

[Disasterpiece]

in template conditionals, you normally use regular php code, so... yeah. Something like this maybe:

<vb:if condition="$vbulletin->session['lastactivity'] > 3600"><span class="idle"><vb:else /><span class="normal"></vb:if>

Not tested, but should do the trick.
Replace 3600 with any amount in seconds after you want the user to be idle.