Had the exact same issue after uninstalling some basic programs through the UI.
Fixed it with
apt-get install sddm-theme-breeze
apt-get install --reinstall plasma-desktop
The first install made it possible to log on again and the second one fixed the rest.
You may use a virtual terminal to login (by pressing ctrl+alt+F1; if that doesn't work, try the other virtual terminals from ctrl+alt+F2 up to ctrl+alt+F6).
for Arc-Theme, in Ubuntu 18.04.
Ubuntu 18.04 change Login Screen theme (not wallpaper) following this Answer and Warnings in it.
I have tried to change login & lock screen themes same as User Shell
Theme Arc-Theme
when you download the arc-theme from https://github.com/horst3180/arc-theme github.
simplfy it like below
this Arc-Theme is lacking content for some keys for Onscreen-Keyboard.
so, i have adapted that from default file /usr/share/gnome-shell/theme/ubuntu.css
copy the lines 1549 to 1566 from /usr/share/gnome-shell/theme/ubuntu.css
and paste under line number 1708 in /usr/share/themes/Arc-Theme/gnome-shell/gnome-shell.css
.keyboard-key.default-key {
border-color: #32322d;
background-color: #20201d;
background-size: 20px; }
.keyboard-key.enter-key {
border-color: #dd4814;
background-color: #e95420;
background-image: url("resource:///org/gnome/shell/theme/key-enter.svg"); }
.keyboard-key.shift-key-lowercase {
background-image: url("resource:///org/gnome/shell/theme/key-shift.svg"); }
.keyboard-key.shift-key-uppercase {
background-image: url("key-shift-uppercase.svg"); }
.keyboard-key.shift-key-uppercase:latched {
background-image: url("key-shift-latched-uppercase.svg"); }
.keyboard-key.hide-key {
background-image: url("resource:///org/gnome/shell/theme/key-hide.svg"); }
.keyboard-key.layout-key {
background-image: url("resource:///org/gnome/shell/theme/key-layout.svg"); }
save the file & close.
now run the below command:
sudo rm /etc/alternatives/gdm3.css && sudo ln -s /usr/share/themes/Arc-Theme/gnome-shell/gnome-shell.css /etc/alternatives/gdm3.css
Example:
pratap@i7-4770:~$ sudo rm /etc/alternatives/gdm3.css && sudo ln -s /usr/share/themes/Arc-Theme/gnome-shell/gnome-shell.css /etc/alternatives/gdm3.css
[sudo] password for pratap:
pratap@i7-4770:~$
now copy the 3 folders common-assets, dark-assets & light-assets from /usr/share/themes/Arc-Theme/gnome-shell/
to /usr/share/gnome-shell/theme/
now copy paste all the content from /usr/share/themes/Arc-Theme/gnome-shell/gnome-shell.css
to /usr/share/gnome-shell/theme/ubuntu.css
file replacing the data in ubuntu.css file.
save the file, close & shutdown
fresh start the system to see the change.
Lock Screen animation:
Login Screen Pictures:
Best Answer
It turns out that for all the themes I've tried needed some qtquick modules that are missing. After trying more themes I realized not all are broken.