TSK's are Windows Mobile theme files that dictate the various system colors and the today background image. I believe they can be placed anywhere on the device, or on the root of the storage card. You can then change themes through Start-->Settings-->Today.
To my knowledge, the color of the top and bottom bars is controlled entirely by the theme unless you override it with a program like UI Tweaker. Are you sure that you didn't previously use a theme that included yellow bars and then manually set the desert image as the today wallpaper?
As an alternative to the program above, there are programs that run natively in OSX:
http://www.pocketmac.net/products/th...mecreator.html
on the PPC itself:
http://www.handango.com/PlatformProd...roductId=24578
and I think I've seen some online theme creators [edit: I think you can create custom themes online here:
http://www.pocketpcscreens.com/].
You can also find lots of new themes at sites like
www.pocketpcthemes.com,
www.wmthemes.com, etc (just google windows mobile themes, pocket pc themes, tsk, etc)
Or use bootcamp/parallels/vmware like me
