Lighthouse/similar scripts don’t work with layout?

I had the Lighthouse JS script installed onto my site, and it worked fine until I switched to a different theme. Then all of a sudden, it stops working (the javascript doesn’t load, so the image just opens up in the same window). I’ve tried reinstalling that script, I’ve tried different Lighthouse scripts, I’ve tried similar scripts like the Thumbnail viewer … but nothing seems to work, even though I’m pretty sure I haven’t done anything wrong in the installation.

Does anyone have any idea what might be wrong? this only started happening after I changed my theme.