Hello Dan,

Thank you for reaching out. To address your comments:

• Indeed currently the ThemeBuilder for UI for ASP.NET Core does not use the latest version of the controls. We have recently introduced the ThemeBuilder Pro and it relies on the Kendo Themes. The Kendo Themes are used across all suites of Kendo components - Angular, React, Blazor, Vue, jQuery, ASP.NET MVC, ASP.NET Core, PHP, JSP and ASP.NET AJAX and as such rely on consistent rendering of the components. This is the reason we have introduced themajor rendering changes with R1 of 2022. The latest release of UI for ASP.NET Core was released with a locked version of the Kendo Themes, so we can ensure the rendering of the UI for ASP.NET Core components is unified with the rest of the suites and clients would be able to use Telerik UI for ASP.NET Core components with the both ThemeBuilder or ThemeBuilder PRO, if additional customization is desired. If further documentation on the ThemeBuilder is needed refer to the ThemeBuilder documentation.
• In regards to the Primary button - this is actually an issue with the configuration used for the Primary button in the ThemeBuilder. I have raised the issue with the responsible team and logged an issue for fixing this, so a primary button is indeed displayed. Unfortunately, the ThemeBuilder repository is private and I cannot share a link to the item.
• The Styling and Appearance article contains a section on the ThemeColor configuration option. You can use the configuration option for all components that expose it, for example you can use the ThemeColor to customize the Loader or the Avatar components, among others. 

I hope the above answers your questions. Feel free to get back to me in case you have any additional questions.

Regards,
Aleksandar
Progress Telerik