Enhance CSS link styles and refactor variable names in site.hs for clarity

assets/css/default.css

@@ -4,6 +4,8 @@
     --color-tag2: darkolivegreen;
     --color-bg: white;
     --color-notice: #fb4f4f;
+    --color-link: #0645ad;
+    --color-link-visited: #551a8b;
 }
 
 html {
@@ -25,9 +27,18 @@ body {
 }
 
 body a {
+    color: var(--color-link);
     text-decoration: none;
 }
 
+body a:visited {
+    color: var(--color-link-visited);
+}
+
+body a[href^="#"]:visited {
+    color: var(--color-link);
+}
+
 body a:hover {
     text-decoration: underline;
 }
@@ -321,18 +332,18 @@ code {
         margin-bottom: 0.5em;
         padding-left: 1em;
         line-height: 1.2;
-        list-style-type: decimal;
-        margin-left: 0
-    }
-
-    div#contents-big ul ul {
         list-style-type: none;
+        margin-left: 0
     }
 
     div#contents-big li+li {
         margin-top: 0.5em
     }
 
+    div#contents-big .toc-section-number {
+        margin-right: 0.2em;
+    }
+
     div#contents-big {
         font-size: 80%;
         padding-top: 0;