Magento 2: How to override gallery.css?

We can override the less file lib in our custom theme. Put this file under web folder:

app/design/frontend/{Vendor}/{Theme}/web/mage/gallery/gallery.less

and add mage/gallery/gallery in PROJECT/dev/tools/grunt/configs/themes.js (or PROJECT/dev/tools/grunt/configs/local-themes/themes.js for magento 2.2) to make grunt compiling it as well.

In your custom config file for Grunt add path to gallery file: 'mage/gallery/gallery' along with 'css/styles-m' and 'css/styles-l'