I'm curious why it's impossible to format a slicer on Excel for Mac? I've created a beautiful dashboard and the slicers are ugly as sin with no way to change the formatting (fill, border, etc.). Any tutorial shows a "Format Whole Slicer" option that does not exist on my menu.
Although it shows screenshots for the PC, the Mac menus and button are very similar and I could follow the steps to modify and reformat slicers. Your picture confirms that you have come as far as duplicating an existing slicer style that you now can modify. More about that in the above link.