You can't do that with a Firefox Theme, which is designed to decorate the toolbar sections of Firefox. not the content area.

Wallpaper is part of the operating system desktop, as is a screensaver. I don't use Chrome so I don't know what is "decorated" in that browser.

See also:

• https://support.mozilla.org/kb/Using+themes+with+Firefox
• http://kb.mozillazine.org/Themes (Lightweight theme)

A Light weight theme (Persona) covers the user interface part of Firefox (toolbar area) and not the browser window with the web pages. Add extra empty toolbars if you want to see more of that theme.
You can create extra toolbars to get extra space via View > Toolbars > Customize
You have to put something on a toolbar (drag a Space item onto it) before closing the Customize window because empty toolbars are automatically removed.

well, that's really not a good answer, you don't have a screen saver then, so you want me to cover my screen with tool bars.