I downloaded and unzipped and checked the theme you mentioned. It is true that its menu is white, but there is nothing special in the manifest.json file, so I can only simply try to modify the manifest.json of my theme in the last few days and submit New version to see if it can solve the problem.
In addition, I searched the link below. Maybe I need to create a theme from scratch instead of customizing it from Firefox Color.
https://bugzilla.mozilla.org/show_bug.cgi?id=1702427

June.15 Update:
I checked the discussion of theme styles in Bugzilla and found the reason for this problem mentioned in the following link:
https://bugzilla.mozilla.org/show_bug.cgi?id=1695990

Then I added "tab_background_text" in the theme you mentioned in manifest.json and temporarily fixed this problem. I still suggest that if you want to use this theme completely, add the CSS code mentioned in the theme description to userChrome.css . If Firefox fixes this problem later, I will update this theme again.
Thanks again for your feedback.

I also thank @mlathrom(https://addons.mozilla.org/en-US/firefox/user/8775692/) for helping me solve this problem where he didn't notice it.:)