In your navbar find
Code:
<td class="alt2" valign="top" nowrap="nowrap"
replace with
Code:
<td class="<if condition="$bbuserinfo[pmunread]">newpmblock<else />alt2</if>" valign="top" nowrap="nowrap">
then just create a class newpmblock in the additional CSS definitions like the one you have done above.
This will change the color of the block in the navbar with your userinfo in to the color as defined in newpmblock from alt2 when there is a new pm.