After I install a template (eg, “hugo -t hyde-x”), how does hugo know which template to use when regenerating content? I don’t know Go well enough to understand the source.
You install themes by placing them (or git cloning) into the /themes directory.
You tell hugo which theme to use by specifying it via either the flag -t or in the config file.
I understand how to install it. What I don’t know is how hugo remembers which theme I’m using after running “hugo -t hugo-x.” I don’t see that hugo is updating my configuration file, so how does it know to use hugo-x later?
It doesn’t. You either add it to the config file yourself (once) or you specify it on the command line every time.
I’d add it to the config file.
It would look like (depending on format)
theme: hugo-x