Register a theme dependencySource:
This function registers a function that returns an
htmlDependency() or list
of such objects. If
session$setCurrentTheme() is called, the function will
be re-executed, and the resulting html dependency will be sent to the client.
A function that takes one argument,
theme(which is a
sass::sass_layer()object), and returns an htmlDependency object, or list of them.
func should not be an anonymous function, or a function which
is defined within the calling function. This is so that,
registerThemeDependency() is called multiple times with the function, it
tries to deduplicate them