use neutral theme for diagrams

the deafult mermaid themes for diagrams don't quite fit the GOV.UK
"brand" colors, and creating the CSS to get the colors pretty is out of
scope, so this just sets it to use the "neutral" (grayscale) theme,
which at least doesn't clash with anything.

lib/govuk_tech_docs/tech_docs_html_renderer.rb

@@ -47,7 +47,7 @@ def block_diagram(code)
         input_path = "#{tmp}/input"
         output_path = "#{tmp}/output.svg"
         File.open(input_path, "w") { |f| f.write(code) }
-        ok = exec_with_timeout("#{mmdc} -i #{input_path} -o #{output_path}", 2)
+        ok = exec_with_timeout("#{mmdc} -i #{input_path} -o #{output_path} --theme neutral", 2)
         if ok && File.exist?(output_path)
           File.read(output_path)
         else