Transparent Background Behind Text

Hi,

I’m using the Mediaphase Lite v 1.0.9 theme. I want to add a transparent black box (maybe 60% transparency) under where it says “Arizona Guns – Buy/Sell/Trade” on the banner portion (right above the 4 buttons on top of the picture). My website link is http://www.azgunbuy.com

I tried various style codes in the box under customize theme but none of them stayed. I would save it and WordPress would automatically remove the code from that box. I’m guessing that I’m going to have to edit one of the php files but I’m not sure what one to edit, what lines to edit, and what to put.

If you look on http://www.azgunbuy.com/forum at the banner image on the top, that is similar to what I’m trying to do. Transparent black background behind the text to make the text stand out more. I can’t modify the image to show this because when I do mobile devices cut off part of the text (responsive theme)

Thank you for any help.