Load the page, open the Edge Dev toolbar and check whether the user agent string is different from Microsoft Edge (Default)
. If it's different you are running the app in compat mode and you can find more details in this articles:
As to the CSS issue check: using the Dev toolbar you can inspect what CSS classes and files are loaded and applied to the combobox elements.
Telerik by Progress
Telerik UI for ASP.NET AJAX is ready for Visual Studio 2017 RC! Learn more.