title: panel seems to forget css styling after importing hvplot.pandas
body: <p>I tried some custom css styling with Panel in a jupyter notebook by following <a href="https://panel.holoviz.org/user_guide/Customization.html#css-classes" rel="nofollow noreferrer">this example</a> from the Panel docs.</p>
  <p>There seems to some weird interference between panel styling and hvplot:</p>
  <p>Importing hvplot.pandas first always seems to make panel ignore later calls to pn.extenstion() with css styling information. (There is a warning about that in the docs. So far so good.)</p>
  <p>When I call pn.extension(...) first it only works with the css specified as 'raw_css'. If specifying the css as 'css_files', a subsequent import of hvplot.pandas makes panel forget the css styling.</p>
  <p>I cannot get panel to use css information from css files and use hvplot.pandas at the same time. Am I missing something?</p>
  <p>(Of course, this may be a corner case for a real application, but it can easily happen in a notebook where you play around with many libraries. And it took me a while to find out that hvplot is involved at all. That's why I'm describing it in some detail here.)</p>
  <p>Here are some code examples:</p>
  <h1>Working as expected</h1>
  <p>In the following examples the Slider is displayed with a yellow background as expected:</p>
  <pre><code>import panel as pn
  pn.extension(raw_css=['.bk.panel-widget-box {background: #f0f000;}'])
  pn.Column(pn.widgets.FloatSlider(name='Number'), css_classes=['panel-widget-box'])
  </code></pre>
  <pre><code>import panel as pn
  pn.extension(css_files=['test.css'])
  pn.Column(pn.widgets.FloatSlider(name='Number'), css_classes=['panel-widget-box'])
  </code></pre>
  <pre><code>import panel as pn
  pn.extension(raw_css=['.bk.panel-widget-box {background: #f0f000;}'])
  import hvplot.pandas
  pn.Column(pn.widgets.FloatSlider(name='Number'), css_classes=['panel-widget-box'])
  </code></pre>
  <p>(Content of test.css: <code>.bk.panel-widget-box {background: #f0f000;}</code>)</p>
  <h1>Not working</h1>
  <p>In the following examples there is no yellow background:</p>
  <pre><code>import panel as pn
  import hvplot.pandas
  pn.extension(raw_css=['.bk.panel-widget-box {background: #f0f000;}'])
  pn.Column(pn.widgets.FloatSlider(name='Number'), css_classes=['panel-widget-box'])
  </code></pre>
  <pre><code>import panel as pn
  import hvplot.pandas
  pn.extension(css_files=['test.css'])
  pn.Column(pn.widgets.FloatSlider(name='Number'), css_classes=['panel-widget-box'])
  </code></pre>
  <pre><code>import panel as pn
  pn.extension(css_files=['test.css'])
  import hvplot.pandas
  pn.Column(pn.widgets.FloatSlider(name='Number'), css_classes=['panel-widget-box'])
  </code></pre>