The core thumbnail styles without any modification look like this:
{F34463272}
This doesn't look every modern, and it is evident that every skin overrides this.
We should look at all skins and simplify the stylesheet to reflect a sensible default state of all of them, rather than rely on skins providing their own CSS overrides.
At minimum we should drop the magnify icon, border, background.
# Goals
[] Skins should have little reason to load the styles and override certain rules by specificity
[] We should revisit Minerva providing its own thumbnail styles and decide whether all skins are required to obey the stylistic rules of https://www.mediawiki.org/wiki/Help:Images#Rendering_a_single_image (see T275201 for more context)
[] The floatleft, floatright, tright and tleft should be reconsidered, and potentially replaced with more meaningful classes (ensuring backwards compatibility)
[] The magnify class is more tightly coupled to the elements that can use it by using a meaningful selector e.g. thumb__magnify instead of .magnify