I currently have the exact same problem previously reported in this bug (where editing the background color of the menu button has no effect), which was closed as fixed. The issue still occurs for me.
UPDATE It turns out the “menu open” style is not automatically selected when clicking “Open Menu” from settings (which is a different bug, I suppose). If I expand the style list, I can see the “menu open” style. Selecting it allows me to change the bg color of the open menu button.
Here is my site Read-Only: LINK