elm_slider: fix theme apply
elm_slider: fix theme apply

Summary:
using theme of the object instead of default for subparts.
@fix

Reviewers: cedric, seoz, Hermet, raster, reutskiy.v.v

Differential Revision: https://phab.enlightenment.org/D2370

tasn/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary,rvandegrift/elementary,tasn/elementary,rvandegrift/elementary