跟以前的情况很像Xfce4桌面背景和桌面图标消失问题解决（伪黑屏）@FreeBSD-CSDN博客，但是以前可以通过使用root账户安装这种方法解决：
pkg install xfce4-desktop这次是因为xfce4 从4.18升级到4.20了，当前pkg库里没有找到xfce4-desktop
尝试解决
查看，发现官网也有人碰到同样的问题：desktop background settings not set (#400) · Issues · Xfce / xfdesktop · GitLab
根据提示执行：
xfconf-query --channel "xfce4-desktop" --list --verbose输出
cat txt/xfconf.log
/backdrop/screen0/monitorDP-2/workspace0/color-style 0
/backdrop/screen0/monitorDP-2/workspace0/image-style 5
/backdrop/screen0/monitorDP-2/workspace0/last-image /usr/local/share/backgrounds/xfce/xfce-shapes.svg
/backdrop/screen0/monitorDP-2/workspace1/color-style 0
/backdrop/screen0/monitorDP-2/workspace1/image-style 5
/backdrop/screen0/monitorDP-2/workspace1/last-image /usr/local/share/backgrounds/xfce/xfce-shapes.svg
/backdrop/screen0/monitorDP-2/workspace2/color-style 0
/backdrop/screen0/monitorDP-2/workspace2/image-style 5
/backdrop/screen0/monitorDP-2/workspace2/last-image /usr/local/share/backgrounds/xfce/xfce-shapes.svg
/backdrop/screen0/monitorDP-2/workspace3/color-style 0
/backdrop/screen0/monitorDP-2/workspace3/image-style 5
/backdrop/screen0/monitorDP-2/workspace3/last-image /usr/local/share/backgrounds/xfce/xfce-shapes.svg
/backdrop/screen0/monitoreDP-1/workspace0/color-style 0
/backdrop/screen0/monitoreDP-1/workspace0/image-style 1
/backdrop/screen0/monitoreDP-1/workspace0/last-image /usr/local/share/backgrounds/xfce/xfce-flower.svg
/backdrop/screen0/monitoreDP-1/workspace1/color-style 0
/backdrop/screen0/monitoreDP-1/workspace1/image-style 5
/backdrop/screen0/monitoreDP-1/workspace1/last-image /usr/local/share/backgrounds/xfce/xfce-shapes.svg
/backdrop/screen0/monitoreDP-1/workspace2/color-style 0
/backdrop/screen0/monitoreDP-1/workspace2/image-style 5
/backdrop/screen0/monitoreDP-1/workspace2/last-image /usr/local/share/backgrounds/xfce/xfce-shapes.svg
/backdrop/screen0/monitoreDP-1/workspace3/color-style 0
/backdrop/screen0/monitoreDP-1/workspace3/image-style 5
/backdrop/screen0/monitoreDP-1/workspace3/last-image /usr/local/share/backgrounds/xfce/xfce-shapes.svg
/desktop-icons/primary true
/desktop-icons/tooltip-size 48.000000
/last/window-height 568
/last/window-width 634
/windowlist-menu/show-submenus true发现没有这个文件/usr/local/share/backgrounds/xfce/xfce-shapes.svg：
ls /usr/local/share/backgrounds/xfce/xfce-shapes.svg
ls: /usr/local/share/backgrounds/xfce/xfce-shapes.svg: 文件或目录不存在原来根本没有/usr/local/share/backgrounds/xfce 这个目录啊！
尝试了寻找xfce4的相关包，但是没有找到。
没有尝试直接cp目录 或者把其它目录的东西cp到目标目录
也许这也是一种办法呢。
采用的办法系统升级+pkg upgrade
先系统升级，从14.1升级到14.2 ，升级之后再进行pkg升级，执行：
sudo pkg upgrade
好么，炸锅了，导致整个xfce4桌面系统都消失了。
经过了曲折的过程：FreeBSD从14.1升级到14.2后xfce桌面无法进入。Fatal server error：xf86OpenConsole:VT_SETMODE VT_PROCESS failed（EE）-CSDN博客最后是使用prots编译安装xfce4-desktop 解决问题。