I have a running instance of Kirby 3 on my laptop (running MAMP). I’m now trying to install a theme and both at the root of the theme as on the panel, I get no errors reported but absolutely no content is served. I checked the .htaccess file and apparently all is correct there. Permissions are OK, so I have no idea as to what I can try to troubleshoot next…
What is the theme you are trying to install and how? Are you trying to merge that into the existing project? Or did you install it as a separate project?
I’m trying to install this theme as a separate project https://creativemarket.com/starckio/3341433-Zêta-for-Kirby-3
Does it come shipped complete with the kirby folder inside it?
Yes, this is what i get after unzipping 
That looks ok apart from the missing .htaccess file but I guess that’s just hidden.
Usually, there should be no extra settings necessary when using MAMP, unless you are using NGINX instead of Apache. Is debugging enabled in the config.php file?
Yes, debug is enabled, and I’m using Apache…
Anything in the error logs, maybe?
If not, you might want to contact the theme author.
hmmm… sorry, which error log?
The PHP error log in the logs folder in Mamp
Problem solved! 
The theme uses a plugin — Uniform — that was not loading properly. I downloaded the plugin again and now all is working. Thank you so much for the help!