rustdoc: horizontal lines are too dark for h3+

[jsha]

Steps to reproduce:

1. Run rustdoc foo.rs on the following file:

/// # Foo
/// ## Foo
/// ### Bar
/// #### Baz
pub fn blah(){}

2. Open doc/foo/fn.blah.html in a browser (in light mode).

Expected results: all horizontal lines under headings have the same grey color.

Actual results: the horizontal line under h3, h4, and h5 is black.

cc @GuillaumeGomez