AbsActivity: Switch between dark and light theme

Imported a whole new "Dark" theme and added a switch to switch between
dark and light. Good for reading at night.

Signed-off-by: Peter Cai <xqsx43cxy@126.com>
TimeLineFragment: Fix hiding and showing logic

Signed-off-by: Peter Cai <xqsx43cxy@126.com>
Styles: optimize shadow color

Signed-off-by: Peter Cai <xqsx43cxy@126.com>