This is a common issue in PHPBB and MyBB forums and I've fixed it before on my own websites.
To fix it, you need to ensure that the style Flat Style is the default for all users including guests and that they have the permission to apply it. Otherwise, guests will be shown the default theme until they sign in.
This is a common issue in PHPBB and MyBB forums and I've fixed it before on my own websites. To fix it, you need to ensure that the style Flat Style is the default for all users including guests and that they have the permission to apply it. Otherwise, guests will be shown the default theme until they sign in.