Control box shadows with theme

It'd be neat to be able to customise the box shadows for breadcrumb, enum-input & context-menu with CSS properties.

Maybe something like:

--vj-shadow: 0 4px 12px rgba(0,0,0,0.5);
--vj-shadow-subtle: "0 4px 12px rgba(0,0,0,0.3);

Submitted #16 if this is desirable 🙂