What exactly are you wanting to do? If something is full opaque it will show the background image. Are you or are you not wanting the background to show through on the form?
I assume you are wanting the form to show a bit of the background but not the textarea or buttons of the form. When you set opacity of an element, it has the effect of giving all of its children the same opacity.
To resolve this, you can use rgba with an alpha of 0.8 with purple the following selector:
You can play around with the alpha to get the value you desire.