Hi Chris,
I tried to reproduce the issue described, however, without success.
What I did is:
1. Opened a new browser window.
2. Log in the vSphere Client and changed the theme to dark.
3. Closed the browser window.
4. Opened a new browser window.
5. I get logged in the vSphere Client and the theme was still dark.
Can you share the steps which you executed in order to reproduce the issue or the best would be if you can share a video so that we can investigate.
Thank you,
Denis