Removing the redundant new local function for style overrides

Bug 1346926: This bug replicates the functionality of the
newly enhanced (in bug 1328310) local/theme directory. I'm
updating the documentation of local/theme at the same time,
to try to avoid this kind of replication in future.

Change-Id: Ia36442ac264f5e0740278592e734ddc0838bb80f