Add dynamic theme-rendering feature & Provide a 'night' theme implemtation

[peter-liu]

1. Add a 'theme' param to http service, to dynamic render to preview page style
2. Provide a 'night' theme implementation

those looks like:

a dark style theme rendered via http param '?theme=night'

[peter-liu]

A new PR submitted to follow ashwinvis's advices. But a little different thought on base.css extracting : https://github.com/instant-markdown/instant-markdown-d/pull/90#discussion_r884065632