Hi Mark,
You set white color of font. Becuase you have that:
body, .sp-preloader
{color: #ffffff;}
what doesn't make sense if your site has white background pages as well.
And by default font was black, and bg white, so you made that change one day.
From what I noticed on some subpages you have dark green background then that white is color. But if you set "white" for body that color will be used every where.
So if on some pages you need different color then in Page > Page Settings > Custom CSS >
body {color: #000;}
this will help.
But if inside your HTML code you pasted/used for example that
<div><span style="color: #ffffff;">Mark is a very knowledgeable and talented dog trainer. From the very first session, he immediately helped to improve our reactive dog’s behavior and she has continued to progress since then. We highly recommend Mark. Thank you!</span></div>
This text will be white anyway, so you have to edit Text addon in HTML mode and remove color tag!
I don't hide the fact that your problem is related to the fact that your site as a whole does not use a dark background, but you have exceptions, which unfortunately causes problems.