ValidationIcon.css

.myValidIcon
{
    width : 20px;
    height : 20px;
    background-color: Green;
}
        
.myErrorIcon
{
    width : 20px;
    height : 20px;
    background-color: Red;
}

ValidationIcon.fs

Controls.Input ""
|> Validator.IsEmail ""
|> Enhance.WithCustomValidationIcon
    { Enhance.ValidationIconConfiguration.Default with
        ValidIconClass = "myValidIcon"
        ErrorIconClass = "myErrorIcon"
    }