You can set the captionBackColor and captionBackTopColor separately (both in JuiceboxBuilder-Pro's 'Customize -> Color' section).
The opacity for each is on a scale from 0 to 1 (0 being fully transparent and 1 being fully opaque).

Unfortunately, the browser within JuiceboxBuilder-Pro does not support CSS3 gradients so the captionBackTopColor will not be rendered in the live preview window (and only the captionBackColor will be visible).
However, it should look fine in all modern browsers so go to 'Gallery -> Preview...' from the drop-down menu at the top and the gallery should be displayed in your system's default browser.
Here is a list of browsers which support CSS3 gradients: http://caniuse.com/#feat=css-gradients

If you have a gallery where this does not seem to be working, please post the URL to your gallery's web page so that I can take a look at the problem for myself and hopefully help further. Also, please let me know what browser(s) you see the problem in. Thank you.