Personally, I find it a bit hard to distinct the two icons (both are just shades of grey), so I suggest that the default forum theme (Insidious) be modified:
Change the "No new posts" icon from the current

to

Change the "New posts" icons (on.png and on2.png) from the current

/

to

Change the "No new posts" legend icon from the current

to

Change the "New posts" legend icon from the current

to
