Thanks,
I searched /usr/share/themes and I did find a icons folder but not any themes folder.
Should I create one and then reinstall the Numix GTK theme?
No. There’s something wrong if you have an “icons” folder in /usr/share/themes. Or maybe you’re talking about /usr/share/icons?
It’s pretty much impossible that the /usr/share/themes folder is missing, as it includes the default MATE themes (which, I assume, still appear in the Appearance software).
Can you check what happens if you list it in the terminal?
ls /usr/share/themes
Also, are you sure that you installed the numix-gtk-theme package and not another numix one? There are a few various packages (icon themes, folder configurator…).
Yes, /usr/share/icons
So, if you run ls /usr/share/themes, am I right to assume that it displays stuff and not a message like “directory does not exist”?
It’s already installed, it was just to confirm that you made a mistake when looking for the themes folder. 
Just in case, try to uninstall/reinstall it:
sudo apt-get remove --purge numix-gtk-theme
sudo apt-get install numix-gtk-theme
Which Ubuntu MATE version are you on, 16.04 or 17.04? in case of 16.04, do you use the PPA to update the MATE desktop to 1.18?
16.04 but right now I am thinking to maybe install 17.04 instead if it’s worthy.
I am a complete Linux noob. Can you explain how to update the MATE desktop to 1.18 ( I did run the Software Updater if it is what you mean) and it is up to date.
On Linux distros, you can use third party repositories which add to the system ones and allow you to install more ercent versions of some packages. They have to be handled with a bit of care but they’re quite useful to have new software.
The Ubuntu MATE developers have a special repository that can upgrade the desktop on 16.04 to a more recent version (1.18). The repo is there: https://launchpad.net/~ubuntu-mate-dev/+archive/ubuntu/xenial-mate
If you want to upgrade your desktop, add the repo and upgrade like this in a terminal:
sudo add-apt-repository ppa:ubuntu-mate-dev/xenial-mate
sudo apt-get update
sudo apt-get dist-upgrade
Note that it only upgrades the desktop itself, the apps in general will still be the old ones from the 16.04 repos. If you want newer apps, you’ll have to install a more recent version of Ubuntu MATE (17.04). With a LTS version (14.04, 16.04, future 18.04…) you can keep it for up to five years but you have older apps. With a regular version (currently 17.04), you have to upgrade your system every six monthes (next version is 17.10 in october) but you have more recent apps.
I see that you checked my message as a solution: did it fix the issue with Numix?
That PPA actually has 1.16.
Oh, yeah, sorry, I probably got confused by my tests of 17.10. Anyway, it’s still a newer version than stock 16.04 (1.12? 1.14?).
@jwegier: if for some reason you want to remove the PPA and revert to the original desktop version, you can do it easily. First, install ppa-purge:
sudo apt-get install ppa-purge
Remove the repo:
sudo ppa-purge ppa:ubuntu-mate-dev/xenial-mate
And then, reboot.
It’ll probably be a good idea to remove it before you upgrade to 18.04 if you stick with the LTS versions.
Thank you, terzag , I have deleted the Maui installation and have installed 17.04 alongside 16.04. Once all is running fine in 17.04 I will install 17.10 instead of 16.04 to see the preview of Ubuntu MATE future release
I have already found some problems after I installed 17.04 and I will open a new thread.
If it’s only to have a preview of a development version, it might be a better idea to do it in a virtual machine rather than installing it for real (unless you want to use it on a daily basis as your main version but it’s not a really good idea).
Ubuntu 17.04 is now my main OS. I will install 17.10 on a small portion of the drive and I may use it whenever I want but not as my main version.
Ok. But then again, in that case it would be a better idea to use a WM: you could use it while still on your main Ubuntu session (no need to reboot) and you wouldn’t need to have to repartition your drive.
Yiu may be right, I will have a look at Oracle VirtualBox
There’s also an opensource version of VirtualBox that’s already in the Ubuntu repos, you just have to install it with:
sudo apt-get install virtualbox
(Warning: if you have Secure Boot on your PC, the vbox module needs to be signed manually or you’ll have to disable SB.)
Thank, I will install VirtualBox. I have never used such a program, so I hope there are a kind tutorial
It’s not that complicated for basic usage: juste choose “new vm”, give it a name, choose “Ubuntu 64 bits”, click next until it asks you to create a virtual hard drive (chose the default vdi option), continue with the default options until it finishes configuration. Start the VM, it’ll ask for a bootable device, just click on the icon next to the dropdown menu to select an ISO and it will boot on it. Installation will be the same as usual. It’ll ask you to format the drive: don’t worry, it’s the simulated drive (the vdi image), it won’t format your real drive.
You can check the VM settings (when it’s not running) if you need specific ones, they’re pretty straightforward.
If you want to install Ubuntu MATE 17.10, be sure to download a daily ISO and not the alpha 2, it has network issues (see there: [Artful] No network after installation).
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Thank you very much for helping