to delete a custom style, initialise the keyboard, go to composer, choose A, then memory clear, exit to composer menu then copy composer A to the custom number you want cleared in Custom Style Copy. I put cleared all customs and single customs on the 7000 book compact disk with full instructions to make it easy for you.
Why? - if you have bought the Technote pre- copied SD cards you will find all customs of related styles, eg all the waltzes, latin, big band etc together so it's very easy to find the right backing for a song by comparing them side by side instantaneously without having to re-load and rely on aural memory. Since you can't necessarily guarantee 20 styles in any one category, a few blanks are neater than some residual unrelated styles.