A GlobalStyleSheet might work for setting page-level CSS attributes, but I am not entirely sure. Typically for control of the page you would define the page however you want in a Jinja2 template, then embed the Bokeh output in the template. You’ll probably need to go that route in any case, since Bokeh’s text-drawing calls to HTML canvas can only use fonts that are loaded on the page, and you will probably have to use a template to explicitly load any extra fonts that are not default in the browser to begin with.
But to really offer more specific advice we would need a complete Minimal Reproducible Example of the approach you have tried that was not working.