title: Background color of bokeh layout
body: <p>I'm playing around with the Bokeh <a href="https://demo.bokehplots.com/apps/sliders" rel="noreferrer">sliders demo</a> (source code <a href="https://github.com/bokeh/bokeh/blob/master/examples/app/sliders.py" rel="noreferrer">here</a>), and I'm trying to change the background color of the entire page. Though changing the color of the figure is easy using <code>background_fill_color</code> and <code>border_fill_color</code>, the rest of the layout still appears on top of a white background. Is there an attribute I can add to the theme that will allow me to set the color via <code>curdoc().theme</code>? </p>