[Expressive design] Update background color for v36 and expressive

Note that it is fine to update v36 now without go/pr-bug because
the non-compatible ag/31875335 is already in 25Q2 system image.

This is a continuation of the reverted ag/33040938 in a backwards
compatible way, in that it updates the background color for v36 but
keeps v31-v35 and expressive unchanged.

Bug: 408996309
Test: manual
Flag: EXEMPT style update
Change-Id: Ia671066ef7c8a282c8194bd9731a5e6a1943f40c