>I maybe very wrong, but in the web world, just installing Bootstrap gives you some basic control over how you controls look. Is there something similar for WPF?
>I don't seem to find anything, I have a very basic little wpf application with a pageframe, buttons, textblocks and textboxes. But it looks very much like something I would have been writing in Winforms. :)
>I was hoping to install some kind of XAML library and voilà, my application would look like something made in the third millenium.
>Was I being a bit naive, and will I have to create my library myself, or are there (simple) libraries around that I can use and would give me a jumpstart?

With WPF/Silverlight, you have complete control over the look and feel of your app. Take a look at Themes, Styling controls and Blend.