With the default window, no. The image file itself has a default opacity to it, so even if you change the window opacity to 255 (which is completely solid), it will still be see through. Fortunately the windows are easy to change. Yanfly has some good ones
here
You'll still need to find a way to change the default opacity though. Yanfly's
Core Engine plugin has an option to do it among many others, and is highly recommended. Otherwise, you could probably find the option location somewhere in the .js files, but I dont know off the top of my head where its located.
If you do opt to use Yanfly's Core Engine, just scroll down to where "Window Opacity" is and change it from 192 to 255.