Is there a way to make Lubuntu 18.10 look like Lubuntu 18.04?

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.

It's because it was a complete rewrite of the desktop. It wasn't just a single underlying technology change; the whole release should be considered a new feature.

The Lubuntu theme, Box, was starting to bitrot. There's no way currently to get that theme back, because not only would it require reworking for LXQt, it needs maintenance.

Sorry; if you enjoyed the old Lubuntu, it's still supported for the next 2.5 years.

You can log into the new LXQt desktop and install LXDE.

In a terminal, type

sudo apt install lxde

It will install lxde and many other packages.

After it finishes, log out. Choose LXDE from the drop down menu in the upper left, and log in again.

It should look more familiar.

Tested in a virtual machine on 10/19/18.

I am going to leave this here even if it is told that it will not be supported coming up with GTK 4. These steps do also install into Xubuntu nicely so you can apply the same look and feel.

EDIT: 2018-10-21 I changed the answer to cover only Lubuntu 18.10 with LXQt and not using LXDE.

I think I understand what you are trying to do here. To get the older look and feel before they changed everything it can be done. First determine what default Lubuntu Art you liked the most and download it from https://launchpad.net/ubuntu/+source/lubuntu-artwork I chose Bionic Beaver 0.71.

Open a terminal window and go to your ~/Downloads folder.

cd ~/Downloads

Now extract the downloaded file.

tar -xvf lubuntu-artwork_0.71.tar.xz

Go to the lubuntu-artwork folder that has been created

cd lubuntu-artwork

In there you will see a bunch of files. All that we are concerned about is the src folder. Go into that folder.

cd src

In there it will contain a usr folder. All we have to do is copy that folder to the root of your drive

sudo cp -Rv * /

It will copy all the files to the correct locations.

Now, click on the Menu -> Preferences -> LXQt Settings -> Appearance

In Widget Style make sure the Lubuntu-dark-panel is selected for both the GTK 2 and GTK 3 theme

Then for Icon Theme choose Lubuntu-dark-panel

Then in LXQt Theme choose Lubuntu

Right click anywhere on the panel at the bottom and choose Configure Panel. Then select Override Icon Theme and choose Lubuntu-dark-panel

PCManFM is not installed by default, but you can still install it after if you like and don't want to use PCManFM-Qt

sudo apt install pcmanfm

PCManFM-Qt (Left) and PCManFM (Right). PCManFM is found under the Menu -> System Tools

Here's what I got the LXQt desktop looking like

Hope this helps!