Improve styles

I've still got more to do, so feel free to leave this pull request open, but I just wanted to check I haven't done anything stupid.

One neat trick I discovered is that I can use a single image, set it as a `Rectangle`'s `OpacityMask`, and then change the `Rectangle`'s `Fill` property to change the colour. Ideally, these images would be WPF drawings so that they'll work in HiDPI mode, but that's for another time.

The main areas left to do are:

* navigator window (I didn't even know this existed until working on these styles!)
* floating anchorable window
* floating document window

Once all that's done, it should be quite easy to add "dark" and "blue" themes.