title: How to setup custom css with bokeh serve
body: <p>How to assign css proprties to a custom class assigned to a widget through <code>css_classes</code> if I'm serving my app through <code>bokeh serve --show</code>?</p>
  
  <pre><code>from bokeh.models import Button
  button = Button(label="Press Me", css_classes=['myclass'])
  curdoc().add_root(button)
  </code></pre>