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" };