Installs new themes, either from an archive (tar.gz or zip) or a copy of the default.

• Requires admin privileges (admin_forum permission)
• Puts themes in $boardurl/Themes.
• Creates a root directory for the theme based on the name of the archive
• Assumes that the theme files and folders are directly inside the archive and not in a directory inside the archive
• Accessed with ?action=theme;sa=install.