DxUpload.ExternalDropZoneDragOverCssClass Property

<div id="overviewDemoDropZone" class="card custom-drop-zone jumbotron">
    <svg class="drop-file-icon mb-3" role="img" style="width: 42px; height: 42px;"><use href="#drop-file-icon"></use></svg>
    <span>Drag and Drop File Here</span>
</div>
<DxUpload Name="myFile" UploadUrl="https://localhost:10000/api/Upload/UploadFile/" 
          ExternalDropZoneCssSelector="#overviewDemoDropZone"
          ExternalDropZoneDragOverCssClass="custom-drag-over border-light text-white">
</DxUpload>

.custom-drop-zone {
    padding: 0 !important;
    border-style: dashed !important;
    border-width: 2px !important;
    height: 230px;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
}
.custom-drop-zone.custom-drag-over {
    background-color: #868A8D;
}
.custom-drop-zone img {
    width: 42px;
    height: 42px;
}

using System.IO;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

[Route("api/[controller]")]
public class UploadController : ControllerBase {
    private readonly IWebHostEnvironment _hostingEnvironment;

    public UploadController(IWebHostEnvironment hostingEnvironment) {
        _hostingEnvironment = hostingEnvironment;
    }

    [HttpPost]
    [Route("UploadFile")]
    // "myFile" is the value of the Upload's "Name" property.
    public ActionResult UploadFile(IFormFile myFile) {
        try {
            // Specify the target location for the uploaded files.
            var path = Path.Combine(_hostingEnvironment.WebRootPath, "uploads");
            // Check whether the target directory exists; otherwise, create it.
            if (!Directory.Exists(path))
                Directory.CreateDirectory(path);
            using (var fileStream = System.IO.File.Create(Path.Combine(path, myFile.FileName))) {
                // Check the file here (its extension, safity, and so on). 
                // If all checks are passed, save the file.
                myFile.CopyTo(fileStream);
            }
        } catch {
            Response.StatusCode = 400;
        }

        return new EmptyResult();
    }
}