How to Highlight Text in Yellow Using Apple Mail

In macOS High Sierra (version 10.13) this is now quite easy. In Mail.app:

First bring up the tool bar, if you already don't have it enabled. This can be enabled by clicking the Tool bar that has the letter "A" on the top right corner of the composing window. Once you do this you can:

1. Select the text you want highlighted
2. Click on the 'Text Background Color' icon. This is shown as a small letter 'a' with a red diagonal line through it.
3. Select a highlight color.

That's all there is to it really.

Note: The complete list of detailed steps with pictures are posted here: http://www.mihalick.us/how-to-highlight-text-in-yellow-using-apple-mail/

Here is a summary of the steps:

1. Obtain some highlighted text in a method of your choosing. I composed an email in MS Outlook on Windows, highlighted some text and then sent that email to one of my accounts in Apple Mail. You can also simply copy some highlighted yellow text from a website page (use this page, if you don't have one handy) by dragging over the text with your mouse and copying to your clipboard.
2. I copied the text out of that email (or the highlighted text above in step 1) and then pasted it into a new document in Apple's TextEdit application.
3. Then in my TextEdit document, I highlighted the text with my cursor, right clicked and selected “Font | Styles…”
4. In the pop-up menu, click ‘Add to Favorites’, and then give your style a name. For example, 'Highlighted Yellow'. I didn’t check either option to include the font or ruler.
5. Click ‘Add’. Your new style for highlighting is now saved and can be used in Apple Mail by the following steps...

Using the style in Apple Mail

1. When you are composing a message in Apple Mail, using your cursor simply highlight the text whose background you want to highlight. Then right click with the mouse and select “Font | Styles…”
2. Ensure the “Favorite Styles” option is selected and then choose your new “Highlighted Yellow” (or whatever you named it) style from the drop down list.
3. Click “Apply” and voila!

For the steps with pictures, you can visit: http://www.mihalick.us/how-to-highlight-text-in-yellow-using-apple-mail/