Unfortunately, modifying the host application interface using CSS is not possible. However, there is a solution using Javascript.
You can utilize
document.documentElement.webkitRequestFullScreen();
to enable full-screen mode. It's important to handle keyboard actions such as ESC and provide users with an exit button for a smooth experience.
For more detailed information, you can refer to the Mozilla documentation. Additionally, it's worth noting that desktop Safari has limited support while iOS Safari does not support this feature at all, so browser compatibility may vary.
It's crucial to understand that this method differs from Kiosk mode, which is used to restrict access to other applications/websites. The full-screen mode discussed here does not offer that level of control.