попробуйте в таком виде:

."</tr><tr><td width=\"100%\" bgcolor=\"#000000\"><img border=\"0\" src=\"themes/images/pixel.gif\" width=\"100%\" height=\"20\"></td>\n"
."</tr><tr><td width=\"100%\" bgcolor=\"#ffffff\"><img border=\"0\" src=\"themes/images/pixel.gif\" width=\"100%\" height=\"20\"></td>\n"