as a user, I want to be able to do some basic styling on the body element of the report

[example notebook that demonstates the issue](https://extremely-alpha.iodide.io/notebooks/26/). To summarize:

- I can't select only the report css without affecting all other css.
- The only method I have to select the report css is really not intuitive.

Start with the example notebook linked in the issue and inspect how report CSS and the body element are currently selected. Clarify the intended scoping and basic styling behavior, then verify that report styling does not affect unrelated CSS and that the selection method is intuitive.

Written by the indexing model from the issue text.