How do you change the theme in the ACP? 

In case of third party themes it is no longer possible since IPS 4.5. The limitation is related to an inaccessible ACP panel in case of outdated or badly designed themes.

Now, the only option available is to swap between the light <> dark default ACP theme.

Edited June 10, 20231 yr by Luuuk

Thanks so much for replying.  Where could I find the setting to change to the dark default ACP theme?

Expand your ACP user menu > Theme:

Oh, wow, that was easy.  Thanks so much.