Hi,

I good idea could be if we can define a "Top Level Class Name" for css class to custom stylesheet produced by themebuilder.

Example:
Image to use one of your standard stylesheet 

if i have

in my page (razor)

   @(Html.Kendo().DropDownList().Name("ControlName")  ... )

that for sure render with the theme loaded in html header section

and after  i put

<div class="MyCustomClassNameForKendoUiWidget">
   @(Html.Kendo().DropDownList().Name("ControlName2")
</div>

that will be render with an different stylesheet loaded in header section of my page and that was produced by your ThemeBuilder


What i obtain is to use different styles for KendoUI widget in same page.