Create a dark theme for the loading screen #8114

Kibana version: 4.5.4

Elasticsearch version: 2.3.5

Description of the problem including expected versus actual behavior:
When I embedded a dark-theme kibana dashboard into my webpage, I expect the loading screen which says "Kibana is loading. Give me a moment here...." is also using the dark color. However, it is still using a light-theme color. It would be nice if it is also using dark-theme color on the loading screen.

Steps to reproduce:

1. Create a dashboard using dark-theme color
2. Embed the iframe in the webpage
3. Open the page