Thanks for your quick reply! I investigated the problem and found out that the php.ini file in my root folder (public_html) did have a max upload file size, but it was set at 128M, so there should not be any problem. Upon further looking around, I found another php.ini file, this one in the wp-admin folder of my installation. This one only had one line, setting error logging to "on". Apparently this was the php.ini file that was causing the problem, because it did not call out any size limit for upload files. I deactivated that file, retried the Pro theme zip file upload, and this time it worked!
So here is another set of questions for you: I do have the license for PRO. I updated from X to X-Pro (is that what you called the previous product iteration) in Nov 2017. The strange php.ini file I found in my wp-admin folder was created at that time. I have not tried to upload any new file to this installation since then. Why was that php.ini file added to the wp-admin folder then? Will I have the same problem when I install then activate the new PRO theme? How about if I add a PRO-child theme?
Now that I have uploaded the PRO theme zip file, I did a preview of the theme, and I could see the way my site will look, which is the same as under the older theme. However, on top of that preview, I got several error messages, or maybe warning messages:
Warning: arraymerge(): Argument #7 is not an array in /home/mrktgu5/public_html/wp-includes/widgets.php on line 1159_
Warning: arraydiff(): Argument #2 is not an array in /home/mrktgu5/public_html/wp-includes/widgets.php on line 1160_
Warning: Invalid argument supplied for foreach() in /home/mrktgu5/publichtml/wp-includes/widgets.php on line 1162_
Warning: arraymerge(): Argument #1 is not an array in /home/mrktgu5/public_html/wp-includes/widgets.php on line 1170_
More questions then: what do these warnings mean? Do I need to fix something before activating the PRO theme? After? Will it affect any features or functions of the theme?
Thanks you very much again for all your help!