If you have a paid plan on your WordPress.com site than you can add custom CSS in the Customiser.
As I was writing that sentence it occurred to me that maybe that is not true for the new block-based themes with full site editing. Maybe it is only possible with classic themes and hybrid themes and not with universal themes.
Putting that little confusion to one side, I noticed today the words ‘See full history’, and if you click that you can see all the CSS you have previously added – and the theme you were using when you added it.
Here’s a more detailed explanation, starting with what you see (if you have a paid plan) when you click on the Customiser.
You see this
And if you click on ‘See full history’, a new page opens and you can see a clickable history of all the revisions, like this:
If you are like me, you might forget what the last theme was that you used, or the one before that. This is a way to find out. As you may be able to see, if you have good eyesight, is that I was previously using the Maywood theme.
Another way to remember what themes you were using previously is to make a note of the themes you use. I write them a post called ‘Do Not Publish This’ that I keep in draft..
Going a step further, you may have noticed that on this site I wrote the name of the current theme in the tag line, like this:
A Diary In Photography | Current theme is ‘Glen’
This is a post about WordPress. Are there no photographs? This is supposed to be a photography site! Where is the elephant?
Ah, here it is
Photograph Works 
I have been working with that coding for many years actually, and have the current configuration backed up in the cloud. I would love to have access to the other files but you have to be on the .ORG side of WP to access it.
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I recall reading somewhere that the files in the .org version are slightly different to the setup in the .com version. Don’t know how true that is because, as you say, we can’t get to the files.
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I’m no expert, but how could they be different!
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Thanks – that got me thinking I had better check because I could be talking out of my hat (an unfordable feeling) – so I asked Support at WP, and this is what he/she said:
So I think you are correct.
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