For actual themes (.dlt files), they just change the color settings, and cannot slow things down.
(Well, unless there is a bug involving combinations of cosmetic settings, but we don't know of any in 12.17.)
Some "theme" posts include complete configuration backups (.ocb files) which can affect pretty much everything, as well as trashing all your preferences. We don't recommend using those, to be honest, although I doubt many of them would affect performance.
There will be an official / built-in dark theme eventually, but we want to overhaul the themes system a bit before that happens.