I think that that's part of the theme. You should be able increase the transparency of the theme entirely. On Windows 7 the page that allows you to control it is called window glass colors and can be accessed via the start menu. Try looking for something like that. Maybe they changed it a bit, giving you more options, but I doubt it.
Just FYI, you can install third-party themes if you get bored of the default Windows themes. Check out this site: http://www.deviantart.com/customization/skins/windows8/visualstyles8/ if you're interested. You'll also need a theme patcher, but there's probably one, or a link to one, within one of the folders of the more popular themes.