appearance: fixed qt theme not take effect if key 'style' not exist

Change-Id: I3e2e641e3854ff39a44cffa69f16c05ee457cf76
tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/65ddf3b1f5b942818613f4a3e0fe1454/