Add css themes

[oknozor]

It would be nice to have some markdown css theme avalaible via a toml config file. see :

• http://markdowncss.github.io/
• http://mixu.net/markdown-styles/#quickstart

[oknozor]

Seems Webkit cannot load css from user dir with <link>...</link>. For now we just copy the stylesheet but it might be better to use this