Adapt Styling for Dark Mode users

[alex-rowden]

New to open-source contributing so apologies if I'm not following protocol correctly. I just started using the tutorial to orient myself to Bokeh and while running through the tutorial I noticed that some of the markup contains style sheets that add colors that aren't suitable for dark mode readers. See "What is in this tutorial" in "01_introduction.py".

This issue is a suggestion to modify the style used to either
a) work in both light and dark model
b) detect the current display settings and change the style appropriately