Getting rid of transparency background in sidebar widget

Here’s the site I’m working on:
https://tinyurl.com/blkumbmfp

I used the text widget in the sidebar to insert a custom navigation menu. But I can’t get rid of the pale green box that appears as a background to the widget, and which disappears when the cursor is over the menu.

I’m trying to remove it totally, but only from the sidebar, not the whole page. If I can’t remove it, I’d like it to use a totally transparent PNG so that it at least is not visible. But so far I have not been able to isolate the code I need to edit.

Please HELP!