To run Visual Studio without all the configurations, themes, and references to user settings, you need to run an instance in a sandbox.
On Windows, open a Powershell or the VS2022 Developer Console:
devenv /RootSuffix NameYouWantToDisplay
NameYouWantToDisplay is the name that will appear on the top-right corner.
Then, you can change the theme by navigating Tools>Options>Environment>General.
More info at Use Visual Studio in Presentation Mode 🔗