Add background to the expander headers in the new resource details panel

[kvenkatrajan]

A follow up suggestions to #1542 :
#1542 (comment)
#1542 (comment)

Add a background to the expander headers for better section visibility.

[JamesNK]

The accordion originally had a lot more stuff to separate sections. Different background colors for headers, borders, etc. I turned it off because it didn't look good.

I've slightly increased the gap between each section (4px to 8px) to help highlight them:

[kvenkatrajan]

For example the background for each section header itself being a different color:

or indented

[leslierichardson95]

I'm team indentation and different header color :)