[NUI] Fix Slider Thumb shown issue (#3187)

[NUI] Fix Slider Thumb shown issue (#3187)

* [NUI] Fix Slider Thumb shown issue

- After default theme update, Slider Thumb disappears when user presses it.
- The reason is that the default resource is not enough,
 and the code is just to get the empty resource. (null value)
- Fixed to show slider thumb for normal, pressed, and disabled states.

Signed-off-by: Seoyeon Kim <seoyeon2.kim@samsung.com>
* Update Slider theme and BorderlineColor

Signed-off-by: Seoyeon Kim <seoyeon2.kim@samsung.com>