Big thing that allowed this degree of personalization and skinning was that everyone was running fairly homogenous display equipment. When you don't need to deal with a whole spectrum of aspect ratios, input affordances, and DPIs, skinning and customization is something you can do in any image editing software.
Correct, and the same goes for webpages, they were built for 15" and/or 17".
It could still be relatively simple if the industry could agree on a few standard DPIs for screens. While still a substantial amount of extra work, raster images with clean 1x, 2x, and maybe 3x variants are a lot more flexible and easier to produce something good looking with than trying to fight vector formats that will end up looking “fuzzy” 70% of the time due to lines landing on half-pixels and such.