How do I change the color of the border from gray to full color via CSS/HTML?

Question

I am a newbie trying to learn HTML CSS

I wanted this border to be completely gray, but as we can see it's half/half - I don't understand why, thanks enter image description here

My console and the picture below:

Answer 1

It may inherit the parent div, please add !important; to force CSS to be applied. Otherwise you can use dev tools to check it (tab styles and calculations)