Starting with v19.2, the ApplicationThemeHelper.SaveApplicationThemeName method saves the application theme name to the %AppData%\{AppName}\{AppName.exe_Url_id}\{version}\user.config file instead of the application configuration file. This allows end users to save the theme when they are not allowed to update the configuration file in the installation folder.
If the user.config file does not exist or does not contain the DXThemeManager section, ApplicationThemeHelper.UpdateApplicationThemeName will apply the theme specified in the application configuration file. So, users without the user.config file will see the previously saved theme as expected.
To revert to the previous behavior and save the theme name in the application configuration file, set the ApplicationThemeHelper.ConfigurationUserLevel property to ConfigurationUserLevel.None.