I tried this
<telerik:RadNumericTextBox CssClass="app-input" ID="txtUnivaPerfTreshold" runat="server"
MaxValue="100" MinValue="1" MaxLength="4" Value="30" EmptyMessage="between %1 - %100" Width="130px" Type="Percent">
</telerik:RadNumericTextBox>
<asp:Button ID="Button1" Text="submit" runat="server" />
and
protected void Page_Load(object sender, EventArgs e)
{
System.Threading.Thread.Sleep(3000);
}
to make sure I see any such styling, but none was added.
Take a look at the surrounding code to see if anything else is affecting the control (custom validators, validation being set in the code-behind, etc.).
Finally, examine the rendered HTML to override the CSS you do not like. I think riError is the class the input will get when invalid, so you can remove borders and any other styling you don't want.