Add a bit of a gradient to the header and maybe the footer

Currently looks like this: 

![CleanShot 2024-03-12 at 16 50 59@2x](https://github.com/simonw/datasette/assets/9599/15ea7809-33c0-4087-bea6-cf904bebfca0)

I've been trying this out on Datasette Cloud for a while and I like it:

![CleanShot 2024-03-12 at 16 51 29@2x](https://github.com/simonw/datasette/assets/9599/65ad6c99-d361-4069-b2a2-83f67f03e025)
