Reduce reliance on theme stylesheet #25

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.