Thank you for the provided images.
In order to change the stylings of the validation tooltip, I would recommend using CSS rules. Here is an example:
The complete implementation could be found in the following dojo example:
Give the approach above a try and let me know if further assistance is needed.
Virtual Classroom, the free self-paced technical training that gets you up to speed with Telerik and Kendo UI products quickly just got a fresh new look + new and improved content including a brand new Blazor course! Check it out at https://learn.telerik.com/.