yeah, this was the classic checkout.
For the focus style I have changed the theme CSS.
For the label I have also changed the CSS on the theme and added the label with one filter.
I needed to add a aria-live region with a snippet for the error announcement to be available when the screen reader was open.
So in total the following is involved:
CSS changes to the theme for the focus style and the label
A small filter on the coupon field to add the label
And a snippet to add the error announcement to an aria-live region
