@Babak i downloaded this...
It's not working... its respond was.
"Oops! We ran into some problems.
Could not process io-dark-mode21600.zip: File does not appear to be a valid add-on archive as expected."
Friend, there's something I learned, which says, when you don't know something, you should ask instead of coming to the wrong conclusions.
I would be happy if I saw a question from you saying: HOW TO INSTALL THIS XENFORO THEME?
Instead of trying the wrong way, come here to tell us that the file is wrong or corrupt while you don't know.
But as this is a family, I'm happy to explain to you, now pay attention:
1 - When we download a xenforo add-on or style (theme) in some forums, they have the habit of putting another folder inside the zip file, this causes an error to appear during installation (This is the main reason for the problem you are facing)
2 - Once you have the ZIP file, check if inside there is the upload folder, if you find it, it means that the file is ready to be installed via the add-on section. If you don't find it, make the upload folder inside the zip file (ZIP ONLY THE UPLOAD FOLDER) then install it.
3 - For xenforo syles (themes), the installation is different from a normal add-on.
3 a) - To install a xenforo theme you need to unzip the zip file on your desktop and inside the folder you will find files such as:
file1.xml
file1_child.xml
and an UPLOAD folder
3 b) Compress the contents of the UPLOAD folder (Pay attention, you cannot compress the upload folder, I said the contents of the UPLOAD folder, please understand)
3 c) upload what you just compressed in part 3b) to the root of the xenforo site and decompress the file there.
3 d) Now go to the administrative area of your xenforo site
Go to APPEARANCE -> SYLES
In the window that opens click on IMPORT
Import from uploaded XML or archive file: (select files from section 3a) file1.xml
file1_child.xml
After that, activate the theme and see what has changed on the home page of the xenforo website