Thank you for sharing this to us!
Being an experimental feature, there are still glitches here and there with Dark Mode in macOS. I found that this was registered in our database already (please refer to it as DEF-4200). Unfortunately, I cannot comment when it will be fixed yet, thus I added your report to our system, hopefully, we will be able to fix it soon!
Thank you once again, and have a great day!
Technical Support Engineer