rustdoc --check-theme doesn't work with minified theme

[Nemo157]

In any crate running cargo doc && rustdoc --check-theme target/doc/light.css will emit a bunch of errors. If you take the css file and run it through something like prettier to get it to an unminified state then --check-theme will work fine.

Testing some manual minification of it one thing it appears to not like is , without a trailing space. Joining the h1, h2, lines into h1,h2, gives

  Missing "h1, h2, h3:not(.impl):not(.method):not(.type):not(.tymethod), h4:not(.method):not(.type):not(.tymethod)" rule

(cc @GuillaumeGomez)