Allow custom root CSS file

[odrotbohm]

I'd like to make local adjustments to the CSS provided out of the box and though I could do so by declaring <stylesheet>foo.css</stylesheet> in my Maven build. foo.css would then @import "site.css"; to pick up the styles defined by the backend and contain a couple of customizations. However, despite the manual definition of the stylesheet, the HTML rendered still refers to site.css, i.e. it doesn't seem to honor the manual override.

[odrotbohm]

It looks like there's a slightly more involved way of hooking in custom CSS.