readthedocs theme: Keep same font-size everywhere
readthedocs theme: Keep same font-size everywhere

The RTD CSS uses both the `html` and `p` selectors to define font size.
MkDocs used to use `body` to reduce size, which only applied to anything
else than `p`. This patch restores compatibility with RTD by using html,
not body. Fixes #801.

peter1000/mkdocs,pjbull/mkdocs,d0ugal/mkdocs,lukfor/mkdocs,d0ugal/mkdocs,waylan/mkdocs,pjbull/mkdocs,lukfor/mkdocs,williamjmorenor/mkdocs,jimporter/mkdocs,mkdocs/mkdocs,peter1000/mkdocs,pjbull/mkdocs,d0ugal/mkdocs,jimporter/mkdocs,jimporter/mkdocs,williamjmorenor/mkdocs,peter1000/mkdocs,williamjmorenor/mkdocs,lukfor/mkdocs,waylan/mkdocs,mkdocs/mkdocs,mkdocs/mkdocs,waylan/mkdocs