Hi Joanne
Thank you for your update.
We have checked the reported behavior by saving the High contrast Black theme as Custom theme, you got an error message in 18.4.0.30 and 18.4.0.39 winform build, but we are unable to observe it and the theme has been properly saved at our end.
Please find the video for the same from following
So could you check it and confirm us whether our replication is same to reproduce it. If not, kindly provide your video that shows the exact issue, it would be helpful for us to check and proceed on it further. Also please confirm your product version details, it would be helpful for us to check and provide you the solution based on it.
Please let us know if you need any other assistance on this.
Regards,
Vijayalakshmi VR