gtk_style_context_add_provider_for_screen (gdk_screen_get_default(), GTK_STYLE_PROVIDER (style_provider), GTK_STYLE_PROVIDER_PRIORITY_USER); https://developer.gnome.org/gtk3/stable/GtkStyleContext.html#gtk-style-context-add-provider-for-screen css data is located at /usr/share/mate-screensaver/lock-dialog-THEME.css, XDG_CONFIG_HOME/gtk-3.0/gtk.css, /usr/share/themes/... /usr/share/mate-screensaver/lock-dialog-THEME.css file has highest priority, if exists: $ gsettings get org.mate.screensaver lock-dialog-theme 'test' $ grep lock-dialog /usr/share/mate-screensaver/lock-dialog-test.css .lock-dialog.background { .lock-dialog frame { .lock-dialog notebook { .lock-dialog entry, .lock-dialog entry:focus { .lock-dialog button, .lock-dialog button:focus { .lock-dialog button:hover, .lock-dialog button:hover:active { ... Otherwise: /usr/share/themes folder if lock-dialog-theme is set to 'debault' $ gsettings get org.mate.screensaver lock-dialog-theme 'default' $ gsettings get org.mate.interface gtk-theme 'TraditionalOk' $ grep lock-dialog /usr/share/themes/TraditionalOk/gtk-3.0/*.css /usr/share/themes/TraditionalOk/gtk-3.0/mate-applications.css:.lock-dialog { /usr/share/themes/TraditionalOk/gtk-3.0/mate-applications.css:.lock-dialog notebook { Priority order: https://developer.gnome.org/gtk3/stable/GtkStyleProvider.html