RossMitchell

Please provide the links to the different pages involved. Also the picture you mentioned.

I routinely setup sites on both localhost and various hosting servers, I just follow the instructions, though the 5 minutes has some poetic license.
However the steps have to be done in the nominated order. I notice that you FTP then admin/install BEFORE the config file and database, bound to end in tears.

I copy/paste everything possible in the text around configuring and setup, nothing gets re-keyed EVER, this includes database names, usernames, passwords etc.

I have the following organisation of local files:

wpsetup/
        core/
             wordpress494AU
             wordpress491AU
        noncore/
                config/
                       wp-config-sample.php
                       wp-config<client>.php (all clients like this)
                themes/
                       storefront224/
                       storefront-child/
                plugins/
                        testimonialslider295/
                z.maintenance
                robotsBAN.txt
                robotsALLOW.txt
                googleWebmasterValidation.html

So that using filezilla I can instigate the core file upload, and then put up selected themes and plugins and housekeeping files too. Then some get renamed.

Setting things up this way is really boring.

Are you really sure you are viewing the site you restored ? Clear cache, try a new computer ?

Of course you backup your theme and plugin files and database before doing anything.

My concern comes from reading suggestions that updating the theme may trash the custom CSS.

If you have edited the .css files of the theme, then these edits will be lost. If these edits were in a child theme they would be safe. If you used the customizer > additional css then these will be safe.

I’m unclear whether that risk is just when changing from Theme A to Theme B or whether there it is likely to affext Theme A v2 to Theme A v3.

Change A-B will not lose settings or files, they will remain in place to be copied, will return to active service if you switch back.
Upgrade V2 to V3 will kill your V2 edits on the parent, a child will survive.

There is the issue that html and css markup classes and IDs etc also hook functions will change between versions, and between themes A to B, so some testing is necessary and revisions anticipated. I have found it handy to be able to review what the old version did etc while tuning the new one.

Yes restoring from an XML backup sounds like a good idea.
When I do a file restore, I put in a small subtle change like setting text red or something so that I know that I have really updated the right file, and that none of the cache things are in the way, afterwards I remove the hack.

Looking at your page with my browser inspector, I see that the video player and styling all involves “mejs” things. Is this a plugin ? All I can suggest is seeking support from this plugin or theme author.

In moving hosting you have changed PHP and other versions. Some hosting control panels permit you to select which version you use.

If you still have access to the old hosting, see which versions it used, create a .php file like:

<?php
 phpinfo();
?>

and save its output.

The quick answer is to choose an existing theme as your parent and then customize it. Effort put into researching which parent to start with is well rewarded as all the features it provides are inherited completely.

When it comes to customizing page templates, start with a copy of the parent’s template file in that role, details on how they are derived and named here:
https://developer.wordpress.org/themes/template-files-section/page-template-files/page-templates/
https://developer.wordpress.org/themes/basics/template-hierarchy/