Split CSS into core and theme

Currently there's a large blob of CSS.

This pull request splits it into two files: *zerobin.css* which does not contain color or border information, and *zerobin-theme-dark.css* which contains exactly those information that was removed from *zerobin.css*

This is an attempt to make ZeroBin more flexible. For my application, ZeroBin is too dark. This PR allows anyone to create a new theme, while keeping the core CSS like fonts, sizes, margins, padding etc.

Merging would be highly appreciated ;-)