With Kendo theme version 8.0.0 the some radius variables like $kendo-border-radius were removed and are no longer present.

Instead, there is now a new feature called kendo-border-radii that automates the creation of different border-radius variables (e.g. like $kendo-border-radius-sm aor $kendo-border-radius-lg)

More about the new variables in Kendo Themes 8.0.0 and above are available in the official documentation:
https://www.telerik.com/design-system/docs/themes/theme-material/theme-variables/#variables 

Consider exposing border-radius variables as they were present in older version but are not inaccessible.