Pygments has poor code block contrast outout using sphinx style.

Change to borland theme. This eliminated 68 poor contrast (< AA)
examples on upgrading.html. Even more on other pages.  axe DevTools is
now happier.

Borland theme/style doesn't color the bacground of the code blocks, so
set to lightyellow in style.css. This still allows good contrast.

Author: rouilj

website/www/_static/style.css

@@ -159,3 +159,5 @@ ul.simple li a { min-height: 48px;
 /* make examples with long lines scrollable */
 div.highlight pre { overflow: auto;}
 }
+/* set background when using borland pygments style */
+div.highlight pre { background: lightyellow;}

website/www/conf.py

@@ -92,7 +92,7 @@
 #show_authors = False
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+pygments_style = 'borland'
 
 
 # Options for HTML output