Adds class "isSPContentPage" to body of webpages that are Sharepoint Content Pages - not so to Settings etc.

isSPContentPage.js

(function(){
  var onDOMReady = function( fn ) {
      if ( document.readyState !== 'loading' ) {
        fn();
      } else {
        document.addEventListener( "DOMContentLoaded", fn );
      }
    },
    markSPContentPages = function(){
      var whitelist = [
            '.welcome', 
            '#ctl00_PlaceHolderMain_WikiField'
          ],
          containsMatchingSelector = function(selector){
             return !!document.querySelector('#DeltaPlaceHolderMain > '+ selector);
          };
      if(whitelist.some(containsMatchingSelector)){
        document.querySelector('body').classList.add('isSPContentPage');
      }
   };
  
  onDOMReady(markSPContentPages);
})();