com/templates/repo/issue/navbar.tmpl

<div class="ui compact left small menu">
	<a class="{{if .PageIsLabels}}active{{end}} item" href="{{.RepoLink}}/labels">{{.i18n.Tr "repo.labels"}}</a>
	<a class="{{if .PageIsMilestones}}active{{end}} item" href="{{.RepoLink}}/milestones">{{.i18n.Tr "repo.milestones"}}</a>
</div>