DOC: fix styling (css) of getting started tutorials

[jorisvandenbossche]

The sphinx theme got some styling updates, but this seems to have broken the custom styling of the getting started guide a bit.

See eg https://pandas.io/docs/getting_started/intro_tutorials/03_subset_data.html : the question marks are not visible.
And there is also some strange green around the code block in the box of installation at https://pandas.io/docs/getting_started/index.html

cc @stijnvanhoey