Hello @Tam Bui,
Thank you for taking time to post this issue in Microsoft Q&A forum.
Probably you enabled Tools > Options > Environment > Accounts > Sign-in options > "Synchronize settings across devices and installs on this machine (enabling this may overwrite local settings)" option.
If so, did you install and use VS 2022 on other machines with the same account signed-in? Did you signed in VS on other machines once but leave it without signing out, and other people are still using that VS with your account signed-in, but change the theme?
Try to check your account connected devices from the Devices page. Kindly check this document: Manage your work or school account connected devices from the Devices page.
Besides, please try to uncheck this option => Tools > Options > Environment > General > Automatically adjust visual experience based on client performance. Not sure for 100%, but maybe on your side, in the specific situation, it will interfere the theme setting.
If the issue persists, feel free to let us know.
Happy coding!
Tianyu
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