title: Adding custom styling to Bokeh Tabs widget
body: <p>Is there a way to add custom styling to the bokeh tabs widget, Specifically I want to change the font size and color for title property under each Panel widget. I have tried a variety of different ways to add styling to the tabs widget on the project I am working on but nothing seems to work. I have tried with this approach <a href="https://stackoverflow.com/questions/55807111/change-style-of-tabs-in-bokeh-plot">Change style of tabs in bokeh plot</a>, <a href="https://discourse.bokeh.org/t/change-design-of-tab-panes/2767/7" rel="nofollow noreferrer">https://discourse.bokeh.org/t/change-design-of-tab-panes/2767/7</a>, and none of the approaches seem to work. 
  I have also tired to use css classes but it seems that they don't work with the Tabs widget.</p>
  
  <p>I preferably would like to keep the current structure of the project; which in this case is just a singular python file that imports different classes pertaining to the project. I was wondering if it's better to refactor the code for this Bokeh app so that it works with an index.html and styles.css page or if it would be better to use a different approach.</p>
  
  <p>I have attached the code block below:</p>
  
  <pre><code>from bokeh.models import (
      TextInput,
      Button,
      Paragraph,
      Div,
  )
  from bokeh.models import (
      Label,
      FileInput,
      Select,
      CheckboxGroup,
      Panel,
      Tabs,
      RangeTool,
      CustomJS,
  )
  from bokeh.plotting import show, figure, curdoc
  from bokeh.models import ColumnDataSource,
  from bokeh.layouts import layout, column, row
  from bokeh.models.widgets import (
      DataTable,
      DateFormatter,
      TableColumn,
      HTMLTemplateFormatter,
  )
  from bokeh.models.widgets import Div
  
  from tornado import gen
  from functools import partial
  
  
  
     # Layout
      tab_dashboard = Panel(
          child=layout(
              [
                  column(select_site, div_value),
                  [div1, div2, div3, div4],
                  data_table,
                  button_download,
              ],
          ),
          title="Export",
      )
      tab_import = Panel(
          child=layout(
              [
                  [column(Div(text="Upload CSV file"), button_upload, div_value)],
                  [div1, div2, div3, div4],
                  data_table,
                  button_import,
              ],
          ),
          title="Import",
      )
  
      tabs = Tabs(tabs=[tab_dashboard, tab_import, tab_settings], margin=[20, 0, 0, 0])
      # add the layout to curdoc
      doc.add_root(layout(textStatus, tabs, sizing_mode="stretch_both"))
  
  </code></pre>
  
  <p><strong>EDIT:</strong> I have also tried the following code snippets that use css_classes but they don't display anything when I call <code>bokeh serve --show app.py</code>.</p>
  
  <pre><code>tab_import = Panel(
          child=layout(
              [
                  [column(Div(text="Upload CSV file"), button_upload, div_value)],
                  [div1, div2, div3, div4],
                  data_table,
                  button_import,
              ],
  
          ),
          title="Import",
      )
      tabs = Tabs(tabs=[tab_dashboard, tab_import, tab_settings], margin=[20, 0, 0, 0], css_classes=["""
  {% block postamble %}
  &lt;style&gt;
  .bk-root .bk-tab {
      background-color: cyan;
      width: 200px;
      color: red;
      font-style: italic;
  }
  .bk-root .bk-tabs-header .bk-tab.bk-active{
  background-color: yellow;
  color: blue;
  font-style: normal;
  font-weight: bold;
  }
  .bk-root .bk-tabs-header .bk-tab:hover{
  background-color: yellow
  }
  &lt;/style&gt;
  {% endblock %}
  """])
      # add the layout to curdoc
      doc.add_root(layout(textStatus, tabs, sizing_mode="stretch_both"))
  </code></pre>
  
  <pre><code>tabs = Tabs(tabs=[tab_dashboard, tab_import, tab_settings], margin=[20, 0, 0, 0])
      # add the layout to curdoc
      doc.add_root(layout(textStatus, tabs, sizing_mode="stretch_both", css_classes=["""
  {% block postamble %}
  &lt;style&gt;
  .bk-root .bk-tab {
      background-color: cyan;
      width: 200px;
      color: red;
      font-style: italic;
  }
  .bk-root .bk-tabs-header .bk-tab.bk-active{
  background-color: yellow;
  color: blue;
  font-style: normal;
  font-weight: bold;
  }
  .bk-root .bk-tabs-header .bk-tab:hover{
  background-color: yellow
  }
  &lt;/style&gt;
  {% endblock %}
  """]))
  </code></pre>