[Feature] Allow users to make the Application Menu transparent

[Lekua]

I'm fairly new to POP OS, previously I was running Linux Mint/Ubuntu. So the transition wasn't hard, but I always had a look I wanted for. I was able to achieve this pretty easily using a theme. But through all my google searching, I couldn't find a way to make the application window transparent. It always has this brown background, which looks nice to some people but I prefer a transparent then a solid background. I think this is a good customization option for users who want it, and I also think we should be able to change the colors in this menu as well.