Excel custom cell style remover
There is not much to review, your code is clean.  There are some things that you can do to improve User Experience (UX).


Replace the label, SelectAllButton and UnselectAllButton with a checkbox. 
Place a Frame around the Controls
Add a ComboBox to change target workbooks
Add buttons to copy and paste styles between workbooks




And if you want to go buckwild you could add a Preview Button