I am assuming that you are using
Modal Popup Extender from
AJAX Control Toolkit. Add below property to your Modal Popup:
BackgroundCssClass="modalBackground"
CSS can be defined in your
CSS Class and referred in the above property.
.modalBackground
{
background-color:
filter:alpha(opacity=70);
opacity:0.7;
}
For more information look at below links:
Modal Popup Extender Basics[
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ModalPopup Extender[
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