fall back to using the default mermaid style

lib/cli.js

@@ -109,6 +109,8 @@ cli.prototype.parse = function(argv, next) {
       } catch (err) {
         this.errors.push(err)
       }
+    } else {
+      options.css = fs.readFileSync(__dirname + '/../dist/mermaid.css')
     }
 
     this.checkPhantom = createCheckPhantom(options.phantomPath)