Create templates or component parts that will allow users to customize the styles of the Upload validation messages through the following selectors:
.k-upload .k-file-validation-message,
.k-upload .k-upload-files .k-file-validation-message,
.k-upload .k-file-success .k-file-validation-message,
.k-upload .k-file-error .k-file-validation-message
Example: https://stackblitz.com/edit/h2vraq?file=src%2Fmain.vue