Dark mode

h2. Story

As a Jira user I would like to be able to change the visual appearance of Jira to a Dark theme, so that my eyes don't cook after staring at Jira all day. 

h2. Details 

h3. Objective

It would be nice to enhance the usability of Jira by reducing eye strain, and creating visual cohesion with the new Mac OS mojave coming up later this fall. 

h3. Visual Changes 

The general dark mode convention is to change all current white space to something dark and neutral, similar to #424242, have any selection areas as a lighter neutral like #F8FAF8, and the highlights to maintain a more tradition blue like #549EFF. 

Text would change from a dark grey/black to a white/eggshell color for readability.