Changing Secondary (Side) Menu to different color

I am trying to change the side menu from black to white, along with the sub-menus. I have set up a Child theme and copied and pasted the secondary menu code into it, but don’t know what part to modify to get it to change to white.

Actually, before the recent Oxygen update, it was working fine and I managed to figure it out, but now I’m lost as to what to do. Also, there was a previous post on this topic, but I think the CSS code has been modified since then.

The website I’m working on is https://red-card.asia.
Any help appreciated on this.