configure/inject backgorund in facebook canvas

I have my game made with unity and with facebook integrated.All works nice, but i cant change background image, i dont know anything about html :). I can do manualy with developer tools in chrome, but i cant do injecting code. i have this code.\

 private void ConfigureCanvas(){
         Debug.Log("CONFIGURE FACEBOOK CANVAS ");
         // INJECT TOP ADS
 
         string injectionAdsFacebook =
             "var headerElement = document.createElement('div'); " +
                 "headerElement.id = ('adsWebFacebook')  ; " +
                 "headerElement.textContent = ('Check out our other great games: ...');" +
                 "headerElement.style.width = 400; " +
                 "headerElement.style.height = 200; " +
                 "headerElement.style.marginLeft = (window.innerWidth - 400)/2;" +
                 "headerElement.style.marginRight = (window.innerWidth - 400)/2;" +
                 "var body = document.getElementById('unityPlayerEmbed');" +
                 "var insertionPoint = body.children[0]; " +
                 "body.insertBefore(headerElement, insertionPoint);";
 
         Application.ExternalEval(injectionAdsFacebook);
 
         // Execute javascript in iframe to keep the player centered and recalculate resolution
         string javaScript = @"
                     window.onresize = function() {
                     var width = window.innerWidth;
                     var height = window.innerHeight;
 
                     var marginX = width * 0.1;
                     var marginY = height *0.1;
 
                     var appHeight = (( window.innerHeight - marginY) / 500) * 500;
                     var appWidth = (appHeight *16) /10; 
 
                     if(appWidth > window.innerWidth){
                         appWidth = (( window.innerWidth - marginX) / 800) * 800;
                         appHeight = (appWidth *10) /16; 
 
                     }
 
                     var unity = UnityObject2.instances[0].getUnity();
                     var unityDiv = document.getElementById(""unityPlayerEmbed"");
 
                     unity.style.width = appWidth + ""px"";
                     unity.style.height = appHeight + ""px"";
 
                     unityDiv.style.marginLeft = (width - appWidth)/2 + ""px"";
                     unityDiv.style.marginTop = (height - appHeight)/2 + ""px"";
                     unityDiv.style.marginRight = (width - appWidth)/2 + ""px"" ;
                     unityDiv.style.marginBottom = (height - appHeight)/2 + ""px"";
                 }
 
                 window.onresize(); // force it to resize now";
         Application.ExternalCall(javaScript);
 
 
 
         string changeCanvasBackground = 
 
                 "var body = document.getElementsByTagName('body');" +
                 "body[0].background-image =  \"url('https://tictacpuzzlesfb.parseapp.com/backgroundsFaceCanvas/spaceCanvasBack.png')\";" +
                 "body[0].background-size = 'cover'";
 
         Application.ExternalEval(changeCanvasBackground);
     }

Well if any can give me a solution, i think this would be pretty easy for a html programmer,if i can i want do with ExternalEval(changaCanvasBackground),every time i get a error like its a bad assigment i try many options and ever get the same error

P.d: sorry for my english, its pretty low :)