Questions[pcmanfm](ubuntu)

I know it is so weird, but last night I set an image on my desktop background and delete the image, then I right-click on the desktop and press the desktop properties to change the style of background (center, stretch and etc) then the desktop properties stopped something like hanging on Windows.

Then a black window alerted something and I press yes without reading it. after that with right-click on the desktop I just see this:

Even some system applications don't work now. for example, I cannot take a screenshot.

Also, my pcmanfm-qt is dead completely, with the command line or clicking on the icon nothing happens.

I don't know what happens. and I have no idea how to fix it.

Since Lubuntu changes its desktop from LXDE to LXQt there have been changes in icons of the panel and PCmanFM, the file manager. I am not an experienced user but I am wondering why a change in technology also has to come with changes in the look and feel of the desktop environment.

Especially when Lubuntu 18.10 is also using PCmanFM (qt version) I don't understand why PCmanFM does not simply look as clean as it did in 18.04.

Is there a way to use settings, e.g. openbox configuration manager or others, to make the new Lubuntu look like the last one(s)?

I have read this link, but there it's talking about nautilus, i am using a different file manager (PCManFm), but i want to add an item to all the right-click menus of the directory.

I have done the following:

added a file ~/.local/share/file-manager/actions/mkthumb.desktop with the following context:

[Desktop Entry]
Type=Action
Tooltip=Generate all the thumbs on the folder
Name=Generate all the thumbs
MimeTypes=inode/directory
Exec=mkthumb.sh -s %u

this doesn't work (i don't even get the menu).

What am i doing wrong, how do i accomplish this? In the event if it's impossible to do this for both Nautilus and PCManFm, i will ask how to do that for the latter.

I can't find the answer to this simple question.

What command should I use to open pcmanfm in a specific directory?
As an example, I want to run: pcmanfm /home/truc/musique.
However, this doesn't seem to work and the manpage isn't very helpful.

I want to change the icons on the lubuntu desktop.