ubuntu18系统美化

1. 将选定的背景图片 login-bg.jpg 移动到 /usr/share/backgrounds/ 目录下

sudo mv currentdir/mypicture.jpg  /usr/share/backgrounds/

2. 修改这个文件sudo gedit /etc/alternatives/gdm3.css，将

#lockDialogGroup {
  background: #2c001e url(resource:///org/gnome/shell/theme/noise-texture.png);
  background-repeat: repeat; 
}

改为

#lockDialogGroup {
  background: #2c001e url(file:///usr/share/backgrounds/login-bg.jpg);         
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center; 
}

3. 保存并重启.