When a RadAlert or RadConfirm is shown without an image (the last argument to their call is an empty string),the radalert/rwAlertDialog and radconfirm/rwConfirmDialog classes go missing from the control's elements (the pairs are for the Classic and Lightweight render modes respectively).
Since these classes are sometimes used to cascade and apply custom styling to this dialog, their removal may be unwanted.
There are several ways to work around this:

-   keep the image, if suitable for your case (i.e., remove the last argument from the RadAlert() call). For example:
RadWindowManager1.RadAlert(text, 330, 150, "", null);

-   use a blank, transparent image instead of no image. It could be 1 transparent pixel (attached here).

-   place the desired style inline in the template. Here is an example for the Lightweight mode:
<AlertTemplate>
    <div class="rwDialog rwAlertDialog">
        <div class="rwDialogContent">
            <div class="rwDialogMessage" style="font-size: 30px;">
                {1}
            </div>
        </div>
        <div class="rwDialogButtons">
            <input type="button" value="OK" class="rwOkBtn" onclick="$find('{0}').close(true); return false;" />
        </div>
    </div>
</AlertTemplate>

-   change the cascade to also affect the other two types of predefined dialogs. For example
.rwDialogText,
.rwDialogMessage
{
    font-size: 30px;
}

-   use a RadNotification to show the message instead: http://demos.telerik.com/aspnet-ajax/notification/examples/servershowwithnewtext/defaultcs.aspx.