Add overrideable variables file, and cleanup less

Before it was mandatory that a theme specify all these variables or else
an error would be thrown complaining about missing variables if using
this prettier plugin. Now themes can opt into overriding such variables.

Author: leoj3n

prettify-variables.less

@@ -0,0 +1,61 @@
+/**
+ * @stylesheet prettify-variables.less Prettify Variables
+ * @parent bit-docs/theme
+ *
+ * @description Variables to customize code highlighting.
+ * 
+ * @body
+ *
+ * Copy these variables into your theme's variables.less to modify.
+ */
+
+/**
+ * @styles color-palette Color Palette
+ *
+ * @description
+ * Variables used to define the site's color palette.
+ * @iframe demos/color-palette.html 630
+ */
+@darkSkies: #484848;
+@haze: #cccccc;
+@fog: #eae9e9;
+@clear: #ffffff;
+@sunrise: #f7f8c3;
+
+
+// Variables used to define color on text elements.
+
+@colorCode: @sunrise;
+
+
+// Variables used to set the color on code syntax.
+
+@str-color: #dd1144;
+@kwd-color: #333333;
+@com-color: #999988;
+@type-color: #445588;
+@lit-color: #445588;
+@pun-color: #333333;
+@opn-color: #333333;
+@clo-color: #333333;
+@tag-color: #navy;
+@atn-color: #teal;
+@atv-color: #dd1144;
+@dec-color: #333333;
+@var-color: #teal;
+@fun-color: #990000;
+
+
+// Variables used to set the color for print or projection.
+
+@str-color-pp: #006600;
+@kwd-color-pp: #006;
+@com-color-pp: #600;
+@typ-color-pp: #404;
+@lit-color-pp: #004444;
+@pun-color-pp: #444400;
+@opn-color-pp: #444400;
+@clo-color-pp: #444400;
+@tag-color-pp: #006;
+@atn-color-pp:  #440044;
+@atv-color-pp: #006600;