test DarkGreen CSS code colour

As a trial, we change the `<pre>..</pre>` and `<code>..</code>`
blocks form "_Indigo_" to "_DarkGreen_".

We needed a colour that was **NOT** on the "_blue_ thru _purple_" colour
range, nor "_red_" that is currently used by links.

We choose "_DarkGreen_" as this is the same colour as the box around
quotes.  Given that `<pre>..</pre>` and `<code>..</code>` text
is unlikely to be confused with a box around a quote, we felt this
was a safe colour to try.

ioccc.css

@@ -413,7 +413,7 @@ pre, code {
     overflow-wrap: normal;
     overflow: visible;
     white-space: pre-wrap;
-    color: Indigo;
+    color: DarkGreen;
 }
 
 a {
@@ -720,7 +720,7 @@ transition: all .3s ease;
     font-weight: bold;
     padding-top: 0;
     padding-bottom: 0;
-    color: Indigo;
+    color: DarkGreen;
 }
 
 /* content text */
@@ -828,7 +828,7 @@ blockquote {
     font-weight: bold;
     padding-top: 0;
     padding-bottom: 0;
-    color: Indigo;
+    color: DarkGreen;
 }
 
 /* footer text */