Hi David, one way to modify a style (text color in this case) in the mobile popup window is to change the property listed below (include it in your css file):
.esriPopupMobile .titlePane {
color: black !important;
}
I added the important rule to override the default property (white). I found the property by running this sample mobile sample in Chrome on the desktop and then inspecting the styles under the 'Elements' tab.
It's also worth noting, you will want to check the browser/device type the user is running your app on. For iOS, you would not want to change the text color or background color.