Step 1. Creat a style sheet file, example.css or open the existing style sheet file.

 

put the following code in the style sheet file:

 
body
{
    text-align:center;
}

 

Step 2. use Editor.EditorWysiwygModeCss Property specify the location of the style sheet that will be used by the editable area. Multiple Style Sheets are supported.

Example: EditorWysiwygModeCss="example.css"

Hope it helps.