First of all, I know there's "background overlay" as per https://support.strikingly.com/hc/en-us/community/posts/207611317-Make-text-box-background-opacity-customisable-?input_string=Opacity
But, 1. that's for an entire background; 2. it's rather dark.
That's 2 separate objectives for me.
To provide a textbox background overlay, it should be the size of the textbox, not the entire background. We try to put landscape Stock photo up there, photos that are of vivid colour. To have a full overlap can really undermine the quality of photo.
Having a "textbox overlay" the size of the entire background doesn't count as a feature for me, it's a "workaround".
Want a Textbox background overlay, not a Background overlay.
There's no way to add layer as background for textbox. There's no way to add photo element that is opaque (in Layout A/B). There's no way to achieve the effect as shown in the screenshot.
Right now the only way to do that is through hardcoded text on an opaque layer in the photo.