Every framework no matter how complete needs to be customizable
however React modules are inherently modifiable, the fact that kendo developers have used java script for building their components makes it very difficult to modify (or even understand) what's under the hood.
*Edit: Here is an screen shot of the contents in node_modules
If anyone wants to modify them, they can downloaded the source code, make the required modification, and used that package instead:
https://www.telerik.com/kendo-react-ui/components/installation/source-code/