Improve markdwon typography

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/gitlab_bootstrap/mixins.scss

@@ -89,10 +89,26 @@
   }
 
   code { padding: 0 4px; }
-  h1 { margin-top: 30px;}
-  h2 { margin-top: 25px;}
-  h3 { margin-top: 20px;}
-  h4 { margin-top: 15px;}
+
+  h1 {
+    margin-top: 45px;
+    font-size: 2.5em;
+  }
+
+  h2 {
+    margin-top: 40px;
+    font-size: 2em;
+  }
+
+  h3 {
+    margin-top: 35px;
+    font-size: 2em;
+  }
+
+  h4 {
+    margin-top: 30px;
+    font-size: 1.5em;
+  }
 
   blockquote p {
     color: #888;
@@ -107,6 +123,16 @@
       background: #EEE;
     }
   }
+
+  code {
+    font-size: inherit;
+    font-weight: inherit;
+    color: #555;
+  }
+
+  li {
+    line-height: 1.5;
+  }
 }
 
 @mixin page-title {