Vue-js-modal: how to set the opacity of overlay？

[mexhfl]

Problem:

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Example & screenshots:

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I have checked stackoverflow for solutions and 100% sure that issue not in my code.

[HoraceKeung]

Did you try setting the colour of the overlay like this?

.v--modal-overlay {
  background: red;
  opacity: 0.9;
}

You can use rgba or hex with opacity (https://stackoverflow.com/a/17239853).
And you will need !important as well.