Add background colour for monokai.sublime theme (#1204)

The monokai.sublime theme does not have a defined background colour.
Monokai themes generally uses a dark greyish colour. In Sublime Text 2 
the value is #272822. This commit assigns that value to `:dark_graphite`
(`:dark` is already in use) and uses this to set a background colour.

lib/rouge/themes/monokai_sublime.rb

@@ -11,6 +11,7 @@ class MonokaiSublime < CSSTheme
       palette :bright_pink    => '#f92672'
       palette :carmine        => '#960050'
       palette :dark           => '#49483e'
+      palette :dark_graphite  => '#272822'
       palette :dark_grey      => '#888888'
       palette :dark_red       => '#aa0000'
       palette :dimgrey        => '#75715e'
@@ -80,7 +81,7 @@ class MonokaiSublime < CSSTheme
             Name::Variable,
             Text::Whitespace,
             Text,
-            Name,                             :fg => :white
+            Name,                             :fg => :white, :bg => :dark_graphite
       style Operator::Word,
             Name::Tag,
             Keyword,