I don’t have a list of the files I have changed. But maybe you can use the development log here to find out the difference between versions. I always tag my releases with a version number, so you should be able to diff between 2.3.8 and the version you are using. The result of a diff can be a bit long though, as it contains all changes; not only style related.
You can also look at individual changesets by clicking on the cog icon next to it.
A tip: make a copy of your current install on your web host or pc (WAMP or XAMPP on windows gives you an Apache web server with PHP and MySQL) and do the upgrade on the copy. That way you can spot the differences.
And if you do your changes again, please do so in a separate css file or in your theme. That way you won’t overwrite your changes on a future update.
