Theres two ways about doing this and I shall list both, ones easier and quicker while the other is more dead on, pick which ever you prefer the most.
Way 1: Go to your projects folder(default is in RMVX under my documents) select the Graphics folder then the System folder, there you'll find a title.png and a windowskin.png or similar files, alter these to change the defaults.
Way 2: Go into the import resources menu and go to Graphics/System and import a new title or Windowskin there.
by default VX only allows one title and one windowskin, but I'm sure there'll be scripts that change this later.
Hope that helps.