Yanfly's Core Engine allows you to muck with the opacity easily.
If you don't want to use a Yanfly plugin, there's also a Hime plugin that can work.
https://www.youtube.com/watch?v=KMArLErZCwQBy default, RPG Maker's window colors are determined by two things1. The windowskin file called "Window" in the img/system folder2. A standard back
himeworks.com
I'm sure there's a line in the code you can alter to do this without a plugin, but altering the code directly is never the best way to go.