function add(sParam, vValue) {
				if (sURL.indexOf("?") != -1) {
					sURL += "&";
				} else {
					sURL += "?";
				}
				sURL += sParam + "=" + vValue;
			}
Returns the appropriate theme, considering the requested theme and the configuration of allowed themes.
If allowed, the requested theme will be returned, otherwise the default theme will be returned.
If either parameter is null, the other will be returned; if both are null, null will be returned.

@private
@param {string} sRequestedTheme
@param {object} oThemeConstraints
@returns {string}