When you try to configure columns in child tables of doctypes (screenshots captured from Educational background section in Employee doctype), the background color of the field is white. This makes the field readable in light mode. In dark mode, you cannot see the text because both text color and background color are white
Configure Column in Light Theme
Configure Column in Dark Theme
After inspecting the input field, I found out the background color of the field is #fff
ERPNext v13.14.0
Frappe v13.14.0