Add sample CSS to header customization example

Signed-off-by: Roger Sheen <roger@infotexture.net>

dev_ref/html-customization-header.dita

@@ -31,6 +31,32 @@
         <cmd>Set <parmname>args.ftr</parmname> to include an XML file as a running footer that appears below the page
           content.</cmd>
       </step>
+      <step importance="optional">
+        <cmd>Add custom CSS rules to style headers and/or footers.</cmd>
+        <info>
+          <p>For example, the DITA-OT documentation stylesheet includes the following header rules:
+            <codeblock>div.header {
+  font-size: 18pt;
+  margin: 0;
+  padding: 0 12px;
+}
+
+div.header p {
+  color: #777;
+  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
+  line-height: 1.3;
+  margin: 0;
+}
+
+div.header hr {
+  border: 0;
+  border-bottom: 1px solid #eee;
+  height: 0;
+}
+</codeblock><!-- Use static excerpt pending 2.4 CI builds -->
+          </p>
+        </info>
+      </step>
     </steps>
     <result>
       <note type="tip">For an example of HTML output generated using this method, see the HTML5 version of the DITA-OT