Thank you, Ingolme.
The answer you provided is precisely what I've been looking for, and will most definitely be of use to me.👍
Now, if I can do the same but have the "Top Row" of the border (Top Left corner, Top Middle and Top Right corner) be of a different height without stretching (as shown in my original post image, the "DLG HEADER BAR"), then it'll really make my day.👌
Have a great day.
P.S. Your reply has also lead me to finding and downloading Firefox Developer Edition , which I'm exploring now.
Border Image Demo.zip