/usr/share/mercurial/templates/monoblue/ is in mercurial-common 4.0-1+deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/mercurial/templates/monoblue/bookmarks.tmpl
/usr/share/mercurial/templates/monoblue/branches.tmpl
/usr/share/mercurial/templates/monoblue/changelog.tmpl
/usr/share/mercurial/templates/monoblue/changelogentry.tmpl
/usr/share/mercurial/templates/monoblue/changeset.tmpl
/usr/share/mercurial/templates/monoblue/error.tmpl
/usr/share/mercurial/templates/monoblue/fileannotate.tmpl
/usr/share/mercurial/templates/monoblue/filecomparison.tmpl
/usr/share/mercurial/templates/monoblue/filediff.tmpl
/usr/share/mercurial/templates/monoblue/filelog.tmpl
/usr/share/mercurial/templates/monoblue/filerevision.tmpl
/usr/share/mercurial/templates/monoblue/footer.tmpl
/usr/share/mercurial/templates/monoblue/graph.tmpl
/usr/share/mercurial/templates/monoblue/header.tmpl
/usr/share/mercurial/templates/monoblue/help.tmpl
/usr/share/mercurial/templates/monoblue/helptopics.tmpl
/usr/share/mercurial/templates/monoblue/index.tmpl
/usr/share/mercurial/templates/monoblue/manifest.tmpl
/usr/share/mercurial/templates/monoblue/map
/usr/share/mercurial/templates/monoblue/notfound.tmpl
/usr/share/mercurial/templates/monoblue/search.tmpl
/usr/share/mercurial/templates/monoblue/shortlog.tmpl
/usr/share/mercurial/templates/monoblue/summary.tmpl
/usr/share/mercurial/templates/monoblue/tags.tmpl