How to highlight text by changing background colour in Mail
Unfortunately, there is no easy way to do this.  Here's how I do it.


⌘ + T: to open up Fonts window
click on the document background color icon and Colors window will open
choose the highlight background color you desire note:

Note: this will change the background color of your entire email

Select the text that you want highlighted and cut (⌘ + X)
Go back to Colors window and change background color of entire email back to original color (usually white).
Paste (⌘ + V) the text back into email, and you will see the text with your desired background color