Add day/night mode button in nav

[DreamTrims]

Аbstract
Add day/night mode button in the top right navigation to make this feature more easily accessible.

Motivation and Purposes
Night mode is really cool but the setting is currently not so easy to access.

** Specification**
Add day/night mode button in the top right navigation to make this feature more easily accessible to all users. Alternative placement could be considered.

Examples and Implementation
Similar functionality found here: luckpool.net (bottom left).