The gtk4 theme option still inherits from gtk.theme for users pinned
before 26.05, but that fallback was previously silent. Move the
default through the shared state-version helper so the compatibility
branch emits the standard deprecation warning and stays consistent
with other future cleanups.
Add a focused test that covers the legacy inheritance path alongside
the existing current-state-version test for the null default.
Signed-off-by: Austin Horstman <khaneliman12@gmail.com>
d3281688bf