Step 1.Once file has been exported - Go to web form folder - open theme folder - open default folder - open css folder - open default.css
step2. Scroll down and find section that contains following code...
/* Theme CSS */
/* This rule will applied to the form container */
#docContainer {
margin: 40px auto;
padding:0px 0px 0px 0px;
width: 600px;
font-family: Helvetica, Arial, Sans-serif;
font-size:13px;
color: #333;
background-color: rgb(246,246,246);
border: 1px solid rgb(193,193,193);
-webkit-box-shadow: 0 0 10px 0 rgba(0,0,0,0.28);
-moz-box-shadow: 0 0 10px 0 rgba(0,0,0,0.28);
box-shadow: 0 0 10px 0 rgba(0,0,0,0.28);
-webkit-border-radius: 6px;
-moz-border-radius: 6px;
border-radius: 6px;
}
step3. change from
margin: 40px auto;
change to
margin: auto!important;
step 4. save default.css file
step 5. upload form files