Usually muse does not allow you to add or change the text highlight color. But with a third-party widget you can achieve that functionality in your muse website. This widget is free for using and not so complicated. Click here to download the widget and double click on the widget to open it in muse, select your text color and background color and voila you are done.
Toufic Nabi is a freelance web designer and developer started his career in 2012 through Upwork. He was nominated as Top-Rated freelancer for three times by Upwork.
Here is Toufic’s Google+.