fix selector constant

js/utils/constants.js

@@ -2,7 +2,7 @@
 export const SLIDES_SELECTOR = '.slides section';
 export const HORIZONTAL_SLIDES_SELECTOR = '.slides>section';
 export const VERTICAL_SLIDES_SELECTOR = '.slides>section.present>section';
-export const SLIDES_BACKGROUNDS_SELECTOR = '.slides>.backgrounds>.slide-background';
+export const SLIDES_BACKGROUNDS_SELECTOR = '.slides~.backgrounds>.slide-background';
 
 // Methods that may not be invoked via the postMessage API
 export const POST_MESSAGE_METHOD_BLACKLIST = /registerPlugin|registerKeyboardShortcut|addKeyBinding|addEventListener|showPreview/;