As far as I can see there is no button available in the HTML Editor that works like the Highlight button in MS Word. You can change the background color, but that involves a few steps including choosing the actual color every time.

 

If there is indeed no such button, I would like to suggest adding it. Basically, you select some text and press the button (or shortcut) and it will change the background color to yellow (or whatever color you chose). The button has a drop down that lets the user select a different color if desired.