How to change background Opacity when bootstrap modal is open

You can override the modal-backdrop opacity in your stylesheet [take note of the .in class]

.modal-backdrop.in {
    opacity: 0.9;
}

http://jsfiddle.net/ThisIsMarkSantiago/r0gwn005/1/

I am assuming you want to set the opacity of the modal background...

Set the opacity via CSS

.modal-backdrop
{
    opacity:0.5 !important;
}

!important prevents the opacity from being overwritten - particularly from Bootstrap in this context.

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