Can we change the colour of border

Answer 1

Yes, you can definitely change the colour of the border d:

Answer 2

Yes like this... " border: 1px solid blue"
of course you decide what color but i put blue in the example, and 1px for the width.

Answer 3

Definitely recommend completing the CSS course based on your questions thus far.
For now:
https://www.sololearn.com/learn/CSS/1101/?ref=app