Hi there,
sorry for all the back and forth. But can you do the following:
1. Remove ALL CSS that you have been provided so far.
2. Leave the CSS Class my-custom-headline
on the headlines you want it applied to.
3. Now the missing ingredient, select the Headlines and highlight the text within, and from the arrow down menu in the block toolbar select Highlight.
Once thats done can you let me know. And ill provide some CSS