As Yahoo [announced] "The user interface on the new beta version (version 5,0,0,1015 and later) does not support older themes. If you try installing an old theme on the new beta version, your toolbar may not look correct."

Update: 22/10/01 there is an updated page out on Yahoo!'s site for [Creating Yahoo Messenger Themes] so if you're a keen themer go check it out :-)

Here's why...
the old tool bar and template for Yahoo Messenger up to and including versions 4,1,0,998

toolbar...


template...

friend_toolbar.bmp
192 pixels wide 135pixels high


now the tool bar and templates for Yahoo Messenger 5,0,0,1015 +


the new "icons" are 48 pixels wide..by 28 pixels high

also floating around is your active IM window "icons"

and then there's

invite.bmp

now divide the width of the original size template (192) by the number of new "icons" (4 as shown below) and you have 48! So any new templates your going to create have to be 48x28 and instead of them all being combined as shown with the "old" template you need to create each one individually. So if you have a toolbar template that isn't working then simply break it down and save as per the file names above (message.bmp, chat.bmp etc)

Notes:
background is still the same 240 x 240pixels
transparent background colour 'magenta' (RGB 255 0 255)
save as 8bit/256 colour
store all themes in seperate folders at the following path..
c:program filesYahoo!messengerskins