I have found this documentation online in order to extract and modify the default template style for telerik controls.
https://docs.telerik.com/devtools/wpf/controls/radrichtextbox/styles-and-templates/styling-radrichtextbox#:~:text=To%20copy%20the%20default%20style%2C%20load%20your%20project%20in%20Expression,Style%20%2D%3E%20Edit%20a%20Copy.
I've done this many times on standard WPF controls so I am familiar with the process. When I open project, I get the below options. There is no option to edit style and edit a copy.
Object -> Edit Style -> Edit a Copy.
https://docs.telerik.com/devtools/wpf/controls/radrichtextbox/styles-and-templates/styling-radrichtextbox#:~:text=To%20copy%20the%20default%20style%2C%20load%20your%20project%20in%20Expression,Style%20%2D%3E%20Edit%20a%20Copy.
I've done this many times on standard WPF controls so I am familiar with the process. When I open project, I get the below options. There is no option to edit style and edit a copy.
Object -> Edit Style -> Edit a Copy.