What finally worked was to go to Appearance/Themes while in my dashboard, and search for the theme from there. Eventually my theme came up, with the option to save and activate it. (It seemed that I had to do this several times; I don’t know why the option never appeared when I initially kept trying to switch back.)
Searching from http://theme.wordpress.com/ did not even generate a result for my theme name. The key seemed to be searching from the dashboard.
“What finally worked” is what I had already implied. As I said, “blogs created before a theme becomes retired aren’t affected”: that is, you go to Appearance > Themes and activate the theme the way you wold activate any other theme.