Change NavBar color to stream color in topic narrow.
Change NavBar color to stream color in topic narrow.

Fix:#514

vishwesh3/zulip-mobile,saketkumar95/zulip-mobile,Sam1301/zulip-mobile,vishwesh3/zulip-mobile,nashvail/zulip-mobile,Sam1301/zulip-mobile,nashvail/zulip-mobile,Sam1301/zulip-mobile,vishwesh3/zulip-mobile,kunall17/zulip-mobile,Sam1301/zulip-mobile,vishwesh3/zulip-mobile,nashvail/zulip-mobile,saketkumar95/zulip-mobile,saketkumar95/zulip-mobile,nashvail/zulip-mobile,kunall17/zulip-mobile,saketkumar95/zulip-mobile,kunall17/zulip-mobile,Sam1301/zulip-mobile,saketkumar95/zulip-mobile,Sam1301/zulip-mobile,kunall17/zulip-mobile