Reduce reliance on theme stylesheet

This is related to #24 but perhaps much easier to implement.
It only requires css rules applied to the Restplain-specific elements.
Specifically, reduce all reliance on theme's `body` styling.

Attached screenshot of Restplain page on site with following style:
```
body{
background-color: black;
color: white;
direction: rtl;
}
```
Not a :happy-face:

IMHO Restplain page should use `user_locale()` and not `site_locale()` but either way, current page is not `i12n`-ized or `l10n`-ized. So it should set `direction: ltr` explicitly. As well as specifically style all 3 main colums.