It's a little late, but perhaps this is useful for you! ;-)

I have updated the default RenderElementAttributes, so that you can pass in a css class, and it handles rendering out cssClasses that are set on the grid elements nicely too.

@functions {
public static MvcHtmlString RenderElementAttributes(dynamic contentItem, string cssClass)
{
    var attrs = new List<string>();
    var classes = new List<string>(); // place to store our classes

    JObject cfg = contentItem.config;

    if (cfg != null)
        foreach (JProperty property in cfg.Properties())
        {
            // if the setting is a class, add it to our classes list
            if (property.Name == "class")
            {
                var propertyValue = HttpUtility.HtmlAttributeEncode(property.Value.ToString());
                classes.Add(propertyValue);
            }
            else
            {
                var propertyValue = HttpUtility.HtmlAttributeEncode(property.Value.ToString());
                attrs.Add(property.Name + "=\"" + propertyValue + "\"");
            }

        }

    JObject style = contentItem.styles;

    if (style != null)
    {
        var cssVals = new List<string>();
        foreach (JProperty property in style.Properties())
        {
            var propertyValue = property.Value.ToString();
            if (string.IsNullOrWhiteSpace(propertyValue) == false)
            {
                cssVals.Add(property.Name + ":" + propertyValue + ";");
            }
        }

        if (cssVals.Any())
            attrs.Add("style=\"" + HttpUtility.HtmlAttributeEncode(string.Join(" ", cssVals)) + "\"");
    }

    // inject the class that was passed into the helper
    if (cssClass != String.Empty)
        classes.Add(cssClass);

    // inject the class attribute
    if (classes.Any())
        attrs.Add("class=\"" + HttpUtility.HtmlAttributeEncode(string.Join(" ", classes)) + "\"");

    return new MvcHtmlString(string.Join(" ", attrs));
}
}

In my site I'm then calling this like:

@helper renderRow(dynamic row)
{
    foreach (var area in row.areas)
    {
    <div @RenderElementAttributes(area, "c" + area.grid.ToString())>
        <div>
        @foreach (var control in area.controls)
        {
            if (control != null && control.editor != null && control.editor.view != null)
            {
                <text>@Html.Partial("grid/editors/base", (object)control)</text>
            }
        }
        </div>
    </div>
    }
}

The important bit being:

@RenderElementAttributes(area, "c" + area.grid.ToString())

This results in:

<div class="c12 m-auto"></div>

For reference the grid config JSON looks like:

[
  {
    "label": "Class",
    "description": "Set a css class",
    "key": "class",
    "view": "radiobuttonlist",
    "modifier": "{0}",
    "prevalues": [
      "",
      "m-auto"
    ]
  }
]

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