How to justify body text and center headings

Changing the CSS code seems to have worked perfectly. Thank you so much for your help, it's much appreciated :-)

You are welcome :)

* I've managed to fix this so I no longer need assistance but can't figure out how to delete this comment now!*

I had succeeded in adding the background color and border to h2 with the code provided to me by Ying above. However, it's no longer showing the color and border.

The code I used is:
.entry-content h2 {
background-color: #bfeaea;
padding-top: 6px;
padding-bottom: 6px;
border: 1px solid #999999;
}

Here is an example post to show that the background colour/border is not appearing:
https://www.staging3.wedreamoftravel.com/colorado-national-monument/

Any idea how to fix it? Thank you!

The CSS seems working fine to me:
https://www.screencast.com/t/rrwj5brOHFj

Did I miss anything?