Don't nod so vigorously you're making the earth shake...
To move style files from Custom button to Memory load button first press Program Menus and then Composer. Then press Custom style copy. Make sure the arrow in the middle is sending stuff in the right direction. Select the custom style you want to copy and click OK.
The original remains where it is but a copy is now where the Memory load button is. Select Memory load button in order to use it.
Don't forget to save that when you are happy with it to your SD Card.