Never mind, it wasn’t the theme that was causing it. The culprit was the ShortPixel Image Optimizer plugin.
It was altering the HTML markup in Final Tiles, so that the IMG tag was translated to PICTURE with a set of SOURCEs, so Final Tiles Gallery couldn’t find the images leading to errors. So it was not a CSS problem.
I deactivated ShortPixel and now everything works. I needed this plugin when I was building the site, but now it’s fulfilled its mission.
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