Thank you for the sent sample.

I found WebForm1.aspx page in your sample. This page has WebGrid control inside. When view this page in browser, WebUI Framework (through Theme architecture) will detect for istheme.config file and automatically apply the contained theme information and component's style definition to the requested page.

As stated in winvista.istheme, the style definition for WebGrid component using DefaultSample.isl (see snippet code of winvista.istheme below).

<?xml version="1.0" encoding="utf-8"?>
<ISTheme>
  <ThemeGroup Name="Default" IsDefault="true">
    <ISComponents>
      <Component Type="ISNet.WebUI.WebDesktop.WebDesktopManager"
        Assembly="ISNet.WebUI.WebDesktop"LayoutName="Vista" DataFile="WinVista.isl" />
      <Component Type="ISNet.WebUI.WebGrid.WebGrid"
        Assembly="ISNet.WebUI.WebGrid" LayoutName="NewStyle" DataFile="DefaultSample.isl" />
    </ISComponents>
  </ThemeGroup>
</ISTheme>

Try to add this data file into WinVista folder. You can use the attached DefaultSample.isl file for test purpose. After that, ensure that you have granted write permission of the WinVista folder.

Save all the changes; build your web application; and view WebForm1.aspx page in browser. WinVista.css file will be generated at first run.

Note: I enclose the generated file, WinVista.css, for reference.

Please let me know if you have different result.

... I have tired all the security settings before. Issue in Windows 10...

I'm using the most recent version of WebGrid (10.0.7200.18) and WebUI.NET Framework 3.0 (3.0.5000.969) and found that the reported problem is not reproducible in Windows 10. Theme architecture successfully generates the css file.

I enclosed the test project and a video which shows how the css is automatically generated by theme architecture in Windows 10. Please find them in the attachment.