Thank you for your input. It is always appreciated.
You are correct (and I fully agree), the Customer should be the first item available for input on an Invoice and will refresh that customer default data after the customer is entered. I will raise an issue to get the Customer set as the first enterable item.
Regarding the "grey" disabled elements, these are intentionally designed to indicate to the person entering information in the screen that these elements are not editable. It is a visual indicator.
The entire application is controlled by a style sheet, so technically it should be possible to modify the style of the application and make it visually whatever you want, in terms of size, colour, bold etc.