Change the opacity of Background for captions

[ecoaster23]

Hi, i would like to find out how i can change the opacity of the background of captions. I understand some one here points out there i can change using the inkdropper icon, but I do not have this icon in my options. Please help. Thank you very much. 

[Sami Succar]

 

[Ann Bens]

You can only change the background on open captions.

Close captions are set.

[ecoaster23]

Thanks guys! 🙂