Not sure if this is the ideal solution, but I'll just throw the option out there anyway. You could build custom field types in place of the defaults provided by Umbraco Forms. You can do stuff like this:

public sealed class SomeSortOfFieldWithAClass : FieldType
{
    [global::Umbraco.Forms.Core.Attributes.Setting("CSS Class", control = "Umbraco.Forms.Core.FieldSetting.TextField", description = "CSS class for dropdown")]
    public string CssClass { get; set; }

    public SomeSortOfFieldWithAClass ()
    {
        Name = "Some Sort of Field with a Class";
        Id = new Guid("5bb40b77-20e8-423c-ac69-ef25599f4601");
        Description = "You can set the class on me!";
        Icon = "dropdownlist.png";
        DataType = FieldDataType.String;
    }

    public override string RenderPreview()
    {
        return "<input type=\"text\"/>";
    }

    public override string RenderPreviewWithPrevalues(List<object> prevalues)
    {
        return RenderPreview();
    }
}

And then, create /umbraco/Plugins/umbracoContour/Views/FieldType.SomeSortOfFieldWithAClass.cshtml. I'd base it off of one of the existing views, but it might look something like this:

@model Umbraco.Forms.Mvc.Models.FieldViewModel

@{
    var mandatoryAttributes = string.Format("data-val=\"true\" data-val-required=\"{0}\"", Model.RequiredErrorMessage);
    var cssClass = Model.AdditionalSettings.First(s => s.Key == "CssClass").Value;
}

<!-- Markup here! -->

It definitely feels gross to have to replace each of the contour field types with your own... In my case, I only needed the css class on my custom field type, so it wasn't a big deal. I'll definitely be following this thread to see if any awesome ideas spring up.

I think this thread has an answer to your question: https://our.umbraco.org/forum/umbraco-pro/contour/60554-Umbraco-Forms-form-specific-markup

Also have a look at: https://our.umbraco.org/Documentation/Products/UmbracoForms/Developer/Custom-Markup/

@Mark Bowser : Thanks for your detailed example. Maybe this is the best solution to my problem right now. But this way, as you said, I need to create a custom field for each type of fields (dropdown, text, radio, textarea, etc.) used in the form. Is your code actually work with Contour or Forms?

@Maria : I've already seen these information, but unfortunately, customizing markup for all the form is not a viable solution since I need to define different css class to different elements of the same type in a form.

@Umbraco Team : Any chance to have a "Field css class" option implemented in the Forms backoffice for each type of fields like the one existing on the form "Styling" section (Form css class)?

Simon, the code I posted was for Contour, but I believe the code is very similar. I'll bet intellisense would work out a lot of the kinks for you. Sorry I don't have an Umbraco Forms example on me.

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