title: Check If Prop Exists in Styled Components
body: <p>I am just getting started with styled components and want to create variations for things like buttons, navbars, etc.  </p>
  
  <p>For instance, I'd like to create a dark version of a navbar (wherein the background color would become dark and the text color light).</p>
  
  <p>What I'd like to do is simply add a prop of dark on a component as follows:</p>
  
  <pre><code>&lt;Navbar dark&gt;...&lt;/Navbar&gt;
  </code></pre>
  
  <p>I'd like to do this as opposed to something like this:</p>
  
  <pre><code>&lt;Navbar type="dark"&gt;...&lt;/Navbar&gt;
  </code></pre>
  
  <p>However, I'm not sure how to do this.  That is, how do I style an element just by checking if the prop name exists (without passing the prop any values)?</p>
  
  <p>Any ideas?  Thanks in advance.</p>