The dark and ayu themes have a menu-like highlight on sidebar items. The
light theme used to, but it was accidentally lost in the sidebar
unification. The change brings back the hover effect in the light theme.
It also makes the hover effect apply consistently to all links in the
sidebar, including headings.
It also simplifies the "In _path_" heading so it's one big link. The
breadcrumbs are still readily available at the top of the page.
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