if you want to load CSS from custom files you need to include into : \Website\Views\Form\Index.cshtml.

This is a content of the file:

@using Sitecore.Form.Core.Configuration
@using Sitecore.Forms.Mvc
@using Sitecore.Forms.Mvc.Html
@model Sitecore.Forms.Mvc.ViewModels.FormViewModel
@{
  ViewContext.ViewData.TemplateInfo.HtmlFieldPrefix = Model.ClientId;

  var styles = new List<string> { "content/themes/base/all.css", "content/Fields.css" };

  var scripts = new List<string>
  {
    "libs/jquery/jquery-2.1.3.min.js",
    "libs/jquery/jquery-ui-1.11.3.min.js",
    "libs/jquery/jquery.validate.min.js",
    "libs/jquery/jquery.validate.unobtrusive.min.js",
    "wffm.min.js",
    "main.min.js"
  };

  if (Sitecore.Forms.Mvc.Settings.EnableBootstrapCssRendering)
  {
      styles.Add("content/bootstrap.min.css");
      scripts.Add("libs/bootstrap/bootstrap.min.js");
  }
}


@Html.RenderStyles(styles.Select(s => Constants.ScriptsBaseUrl + s).ToArray())
@Html.RenderStyles(string.Format(StaticSettings.CustomCssPath, string.Empty))
@Html.EditorFor(x => Model)
@Html.RenderScripts(scripts.Select(s => Constants.ScriptsBaseUrl + s).ToArray())

You can modify this line to add your custom css file :

var styles = new List { "content/themes/base/all.css", "content/Fields.css" };

if you want to load CSS from custom files you need to include into : \Website\Views\Form\Index.cshtml.

This is a content of the file:

@using Sitecore.Form.Core.Configuration
@using Sitecore.Forms.Mvc
@using Sitecore.Forms.Mvc.Html
@model Sitecore.Forms.Mvc.ViewModels.FormViewModel
@{
  ViewContext.ViewData.TemplateInfo.HtmlFieldPrefix = Model.ClientId;

  var styles = new List<string> { "content/themes/base/all.css", "content/Fields.css" };

  var scripts = new List<string>
  {
    "libs/jquery/jquery-2.1.3.min.js",
    "libs/jquery/jquery-ui-1.11.3.min.js",
    "libs/jquery/jquery.validate.min.js",
    "libs/jquery/jquery.validate.unobtrusive.min.js",
    "wffm.min.js",
    "main.min.js"
  };

  if (Sitecore.Forms.Mvc.Settings.EnableBootstrapCssRendering)
  {
      styles.Add("content/bootstrap.min.css");
      scripts.Add("libs/bootstrap/bootstrap.min.js");
  }
}


@Html.RenderStyles(styles.Select(s => Constants.ScriptsBaseUrl + s).ToArray())
@Html.RenderStyles(string.Format(StaticSettings.CustomCssPath, string.Empty))
@Html.EditorFor(x => Model)
@Html.RenderScripts(scripts.Select(s => Constants.ScriptsBaseUrl + s).ToArray())

You can modify this line to add your custom css file :

var styles = new List<string> { "content/themes/base/all.css", "content/Fields.css" };