Простым поиском по apt видно пакеты, которые начинаются с фразы "gnome-shell-extension":
Сортировка… Готово
Полнотекстовый поиск… Готово
gnome-shell-extension-appindicator/bionic,bionic 18.04.1 all
App indicators for GNOME Shell
gnome-shell-extension-autohidetopbar/bionic,bionic 20171126-2 all
GNOME shell automatic topbar hider
gnome-shell-extension-better-volume/bionic,bionic 0.0-git20170425.1a66fab-1 all
GNOME Shell extension that improves volume control
gnome-shell-extension-caffeine/bionic,bionic 0~git20171229-2 all
GNOME Shell extension to keep your computer awake
gnome-shell-extension-dash-to-panel/bionic,bionic 13-1 all
combines the dash and the GNOME main panel into a single panel
gnome-shell-extension-dashtodock/bionic,bionic 63-1 all
dash-to-dock extension for GNOME shell
gnome-shell-extension-disconnect-wifi/bionic,bionic 17-2 all
disconnect wifi extension for GNOME shell
gnome-shell-extension-hard-disk-led/bionic,bionic 19-1 all
Shows harddisk activity (IO speed read/write and LED) in GNOME Shell
gnome-shell-extension-hide-activities/bionic,bionic 0.00~git20131024.1.6574986-1 all
GNOME shell extension that hides the activities button
gnome-shell-extension-hide-veth/bionic,bionic 1.0.2-1 all
hides veth devices typically used by docker and lxc
gnome-shell-extension-impatience/bionic,bionic 0.4.5-3 all
speed up the gnome-shell animation speed
gnome-shell-extension-log-out-button/bionic,bionic 1.0.7-1 all
Adds a log out button to the system action list in GNOME Shell
gnome-shell-extension-mediaplayer/bionic,bionic 3.5-3 all
GNOME Shell extension to control media players
gnome-shell-extension-move-clock/bionic,bionic 1.01-1 all
move clock extension for GNOME shell
gnome-shell-extension-multi-monitors/bionic,bionic 0.00~git20171014.1.df5d6e4-1 all
Better support for additional monitors in GNOME shell
gnome-shell-extension-no-annoyance/bionic,bionic 0+20170928-f21d09a-1 all
removes GNOME 'Window is ready' notifications
gnome-shell-extension-onboard/bionic,bionic 1.4.1-2ubuntu1 all
GNOME Shell extension for the on-screen keyboard Onboard
gnome-shell-extension-pixelsaver/bionic,bionic 1.10+git20161217-49f47bf-1 all
pixel saver extension for GNOME shell
gnome-shell-extension-redshift/bionic,bionic 3.20.1-2 all
redshift extension for GNOME Shell
gnome-shell-extension-remove-dropdown-arrows/bionic,bionic 9-1 all
removes drop down arrows from panel on GNOME shell
gnome-shell-extension-shortcuts/bionic,bionic 1.1.1-1 all
Creates a shortcuts help pop-up in GNOME Shell
gnome-shell-extension-show-ip/bionic,bionic 8-3 all
Shows the current private or public IP address
gnome-shell-extension-suspend-button/bionic,bionic 0~git20171025-2 all
Gnome-shell extension to modify the suspend/shutdown buttons
gnome-shell-extension-system-monitor/bionic-updates,bionic-updates 35-1ubuntu0.18.04.1 all
Display system information in GNOME Shell status bar
gnome-shell-extension-taskbar/bionic,bionic 57.0-1 all
TaskBar Extension for GNOME Shell
gnome-shell-extension-tilix-dropdown/bionic,bionic 5.1-1 all
launch tilix in quake-mode from gnome-shell
gnome-shell-extension-tilix-shortcut/bionic,bionic 1.0.1-2 all
Adds easy to use configurable keyboard shortcut for tilix
gnome-shell-extension-top-icons-plus/bionic,bionic 21-3 all
GNOME Shell extension to move system tray icons to top bar
gnome-shell-extension-trash/bionic,bionic 0.2.0-git20161122.ad29112-1 all
trash applet for GNOME shell
gnome-shell-extension-ubuntu-dock/bionic-updates,bionic-updates 0.9.1ubuntu18.04.3 all
Ubuntu Dock for GNOME Shell
gnome-shell-extension-weather/bionic,bionic 0~20170402.git34506a6-1 all
weather extension for GNOME Shell
gnome-shell-extension-workspaces-to-dock/bionic,bionic 44-1 all
additional options for GNOME workspace switcher
gnome-shell-extensions/bionic,bionic 3.28.0-2 all
Extensions to extend functionality of GNOME Shell
gnome-shell-extensions-gpaste/bionic,bionic 3.28.0-2 all
GPaste extension for GNOME Shell
Подписи под каждым пакетом - это краткое описание. Пакета же gnome-shell-extensions-User_Theme в штатном репозитории 18.04 я не наблюдаю. Если он в каком-то ином репозитории - то этот репозиторий сперва нужно подключить. Если же его вовсе не существует (взяли из головы, грубо говоря), то и apt его не найдет ни каким образом... Что за расширение хотите поставить?