Reply #1 Top
Use the per-skinning feature to exclude it from being skinned (change the look of a particular application). Find all the yahoo exe's you can and add them to the do not skin list.
Reply #2 Top
In a bit more detail on Bebi's suggestion:

Open the WindowBlind Configuration screen and click on "Change the look of a particular application" and then on "add per application setting". Locate the .exe of yahoo messenger and select it. Under "Total exclusion" put a check mark in "Ignore this application and do not skin it". Click OK.