By default, RPG Maker’s window colors are determined by two things
1. The windowskin file called “Window” in the img/system folder
2. A standard background opacity defined in the code
So for example, the default windows are dark colored with a semi-transparent opacity.
However, what if you just wanted to have a nice solid color instead of the translucent dark?
Maybe like this solid blue?

Using this plugin, you can quickly set up the colors for your windows.
This plugin overwrites the background color/texture settings of the windowskin.