Most of my skins do
not crop the thumbnails, as long as you don't choose the "fixed shape thumbnails" option -
Jupiter,
Mars,
Mercury,
Neptune,
Pluto,
Saturn. Pick a planet.
Both
Jupiter and
Saturn offer the "square box with non-fixed-shape thumbnail" effect. See, for example:
https://jgromit.com/jupiter/surround/ (a non-lightbox skin).
https://jgromit.com/saturn/music/ (a lightbox skin).
In each case, you have complete control over borders and spacing (all in the settings - do not start trying to code-hack - it's not necessary).
In the other four, non-fixed-shape thumbnails are presented in a justified gallery. They do not crop the thumbnails unless the aspect ratios are extreme, but you can always tell the skin not to constrain the aspect ratios at all. You can control spacing, but borders are limited to 1px.
Browse:
https://jgromit.com/