Rated 4 out of 5 stars

Earlier version had a few problems: I was not sure what addons or font setting I had interefered with Walnut theme, and so I gave up. (Some fields such as "Cc:", "Reply-to*" labels when I compose a new mail in the address section was invisible, or transparent and was unusable.

This version seems to work rather well. Great!

However, I didn't give five stars because I share the following sentiment of BJ also.

So, if we have a settable option to change the color of the background ( panels ?), it would be perfect!
Color and patterns even.

I prefer lighter color as used in the folder pane as of today. I am afraid this contrast problem can be understood only when one gets older...

BJ said:
>> I applaud the author for making a noble effort; however, these middle aged eyes require more contrast and much less busy-ness.

> by BJ -

This review is for a previous version of the add-on (1.8.48). 

My other themes

One could also try my other themes: Walnut2 or Nautipolis. The first has also a wood theme, but the second has more contrast (using the default system colors).