Here's a 6 pack skin installer for Mac users, this will save the trouble of Manually doing it.
The Skin bmp's install to /Applications/Band-in-a-Box/Band-in-a-Box.app/Contents/Resources/MainScreenButtons.
Just re-apply after any updates.
Let me know if they work ok for you.

https://www.dropbox.com/s/2ua25ghnyqwnqj4/Biab_Skins_Pack.dmg?dl=1

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ReaTrak Biab & Plugin in Reaper
Free Biab Chord Picker Tool