/usr/share/ubiquity/gtk/ is in ubiquity-frontend-gtk 18.04.14.
This file is owned by root:root, with mode 0o755.
..
/usr/share/ubiquity/gtk/gtkwidgets.xml
/usr/share/ubiquity/gtk/nm.xml
/usr/share/ubiquity/gtk/stepKeyboardConf.ui
/usr/share/ubiquity/gtk/stepLanguage.ui
/usr/share/ubiquity/gtk/stepLanguageOnly.ui
/usr/share/ubiquity/gtk/stepLocation.ui
/usr/share/ubiquity/gtk/stepPartAdvanced.ui
/usr/share/ubiquity/gtk/stepPartAsk.ui
/usr/share/ubiquity/gtk/stepPartAuto.ui
/usr/share/ubiquity/gtk/stepPartCrypto.ui
/usr/share/ubiquity/gtk/stepPrepare.ui
/usr/share/ubiquity/gtk/stepUserInfo.ui
/usr/share/ubiquity/gtk/stepWireless.ui
/usr/share/ubiquity/gtk/ubiquity.ui