Hello,
I added a custom style per below:
.highlight {background-color: yellow;}
This became available in the dropdown menu. Within that menu, the text had a background colour of yellow. I had some content highlighted and selected this option from the menu. In the editor, the yellow background colour did not appear. I published the page then viewed the published page. The class name was applied to a SPAN tag around the content I had selected, but the actual styling (background colour) was not applied.
Is this simply because I was testing on the demo site? Have I missed something?