Don't use `documentElement` to set/update `--bokeh-color-scheme`

Follow-up for PR https://github.com/bokeh/bokeh/pull/14915

The current approach to set/update the CSS variable that handles the current color-scheme value (--bokeh-color-scheme) uses documentElement to do so. A better approach should be found (one that allows to set the color-scheme value per Bokeh document)