Alternate themes for the dashboard

People may prefer different themes for the dashboard. This both allows individual style to come out, and also exposes a nice laboratory for others to explore and create better visual style than we can. (which we can then steal as defaults).

There are a few places where we currently specify colors and styling:

1. In the bokeh [theme.yaml](https://github.com/dask/distributed/blob/master/distributed/bokeh/theme.yaml) . This determines the style of the bokeh figures, except what we specify in code
2. In the code, where we specify colors explicitly (for example we currently have hard-coded values for "blue", "red" and "Viridis11" within our codecase. See https://github.com/dask/distributed/blob/a78109e3605756562dc7caed4608db5e99b8dbd0/distributed/bokeh/scheduler.py#L441-L444
3. In [static/css/base.css](https://github.com/dask/distributed/blob/master/distributed/bokeh/static/css/base.css) where we specify background color, and color for the navbar

Currently if you want to build a new theme you'll need to touch all of these. Once you've built that theme it's not easy to switch back and forth.

What is the right way to represent styling so that people can easily experiment with it and switch back and forth without having to repackage things?

cc @canavandl