Hi @heatherruthcreative,

I hope you are well today and thank you for your question.

You can try changing it by adding the following CSS code in the Custom CSS code option of your theme on the below path.
Admin Area -> Appearance -> Theme Settings -> Custom Code -> CSS Code

::selection { background: #000; }

::-moz-selection { background: #000; }

::-webkit-selection { background:#000; }

You can change the color value in the above code to whatever you want to use by referring the following pages.

http://www.w3schools.com/html/html_colors.asp
http://www.w3schools.com/html/html_colorvalues.asp
http://www.w3schools.com/tags/ref_colorpicker.asp

Please note that only Safari and Firefox are supporting this and some of the most recent version of chrome browser.

Best regards,
Shufflehound team