I have written about this issue before without resolution. Using the prebuilt template, my form displays properly in build mode. The font colour changes when the form goes live. I have received numerous complaints from customers that it is difficult to read.
I cloned your form and I can see that the text you're referring is hard to see.
Here's a guide: How-to-Inject-Custom-CSS-Codes. Keep in mind that you can change the color hex code to your own preferences.
Thank you for the detailed response, problem solved!