Remove code duplicated from minima

This makes the minima dependency less fragile hopefully minimizing issues such as were encountered last time minima updated.  New method for incorporating the modified syntax-highlighting from jekyll/minima#79.

Author: adamvoss

_sass/minima.scss

@@ -18,6 +18,11 @@ cyan      #2aa198  strings, numbers
 green     #859900  operators, other keywords
 */
 
+.highlight{
+    .highlighter-rouge & {
+        background: #fdf6e3;
+    }
+}
 .highlight { background-color: #fdf6e3; color: #586e75 }
 .highlight .c { color: #93a1a1 } /* Comment */
 .highlight .err { color: #586e75 } /* Error */

_sass/minima/_syntax-highlighting.scss

@@ -5,14 +5,15 @@
 
 // Import partials from the `minima` theme.
 @import "minima";
+@import "minima/syntax-highlighting";
 
 // Import navigation partial for Software page
 @import "intra-navigation";
 
 // _base.scss overrides
 pre,
 code {
-  background-color: inherit;
+  background-color: unset;
 }
 
 .wrapper {