CSS styling of native (inbuilt) wordpress thumbnails [closed]

The gallery_shortcode with its default styles can be found in wp-includes/media.php line 906 to 1089. There is also an article in the codex that might be useful: Gallery Shortcode. There is also a couple of filters available as you can see from the source code that you can use

The gallery shortcode uses the default thumbnail size, which is 150 x 150 px. This can be changed like below to use the medium size thumbnail

As for the responsiveness, you are going to have to figure out something there. I don’t have any time to code now or test, but it might become necessary to create your own shortcode, because, AFAIK, the type of responsiveness does not exists in the current shortcode. I would think that you will need to strip the complete section of code from the source code which handles adding columns to the gallery

I hope this helps you in some constructive way in solving your issue

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