There is a stylelint rule to check for missing end of source newline, but css with two or more newline at the end are not reported to be reduce to one newline
See https://gerrit.wikimedia.org/r/#/c/348509/2/modules/skinOverride.css for an example file, where it was not reported
We're currently tracking issues on https://www.mediawiki.org/wiki/Manual_talk:Coding_conventions/CSS or https://github.com/wikimedia/stylelint-config-wikimedia/issues for the keen; follow-up there?
(Also, I don't think stylelint allows us to set max-empty-lines to 0 at the end of the file but 1 everywhere else…)