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.
registerThemeDependency(func)A function that takes one argument, theme (which is a
sass::sass_layer() object), and returns an htmlDependency object, or list
of them.
Note that 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