/usr/share/javascript/bootstrap/less/mixins/ is in libjs-bootstrap 3.3.7+dfsg-2.
This file is owned by root:root, with mode 0o755.
..
/usr/share/javascript/bootstrap/less/mixins/alerts.less
/usr/share/javascript/bootstrap/less/mixins/background-variant.less
/usr/share/javascript/bootstrap/less/mixins/border-radius.less
/usr/share/javascript/bootstrap/less/mixins/buttons.less
/usr/share/javascript/bootstrap/less/mixins/center-block.less
/usr/share/javascript/bootstrap/less/mixins/clearfix.less
/usr/share/javascript/bootstrap/less/mixins/forms.less
/usr/share/javascript/bootstrap/less/mixins/gradients.less
/usr/share/javascript/bootstrap/less/mixins/grid-framework.less
/usr/share/javascript/bootstrap/less/mixins/grid.less
/usr/share/javascript/bootstrap/less/mixins/hide-text.less
/usr/share/javascript/bootstrap/less/mixins/image.less
/usr/share/javascript/bootstrap/less/mixins/labels.less
/usr/share/javascript/bootstrap/less/mixins/list-group.less
/usr/share/javascript/bootstrap/less/mixins/nav-divider.less
/usr/share/javascript/bootstrap/less/mixins/nav-vertical-align.less
/usr/share/javascript/bootstrap/less/mixins/opacity.less
/usr/share/javascript/bootstrap/less/mixins/pagination.less
/usr/share/javascript/bootstrap/less/mixins/panels.less
/usr/share/javascript/bootstrap/less/mixins/progress-bar.less
/usr/share/javascript/bootstrap/less/mixins/reset-filter.less
/usr/share/javascript/bootstrap/less/mixins/reset-text.less
/usr/share/javascript/bootstrap/less/mixins/resize.less
/usr/share/javascript/bootstrap/less/mixins/responsive-visibility.less
/usr/share/javascript/bootstrap/less/mixins/size.less
/usr/share/javascript/bootstrap/less/mixins/tab-focus.less
/usr/share/javascript/bootstrap/less/mixins/table-row.less
/usr/share/javascript/bootstrap/less/mixins/text-emphasis.less
/usr/share/javascript/bootstrap/less/mixins/text-overflow.less
/usr/share/javascript/bootstrap/less/mixins/vendor-prefixes.less