There is a problem with webmin themes. As we know, there are 2 menus for changing the theme. If we use the Menu>Webmin>Change Language and Theme, it will change the theme for all menus but not for the login screen.
If we use the Menu>Webmin>Webmin Configuration>Webmin Themes, it will not change the theme for all menus but will change the theme for the login screen.
They work independently.