function loadTheme(id) {
        var extension = extensions[id];
        if (extension.installInfo && extension.installInfo.metadata && extension.installInfo.metadata.theme) {
            ThemeManager.loadPackage(extension.installInfo);
        }
    }
@private
Verifies if an extension is a theme based on the presence of the field "theme"
in the package.json.  If it is a theme, then the theme file is just loaded by the
ThemeManager

@param {string} id of the theme extension to load