In the “Migrating the first version of a template or a module from a local wiki to the global repo” section it says
Some others, however, will be in Common.css or other wiki pages. Copy them to the global repository.
For me it means that Common.css styles should be copied to Common.css in the global wiki. They shouldn’t. Common.css of the global wiki shouldn’t be global—globalizing it causes yet another software component to develop, maintain, secure and provide with adequate tooling (e.g. possibility to preview its changes on different wikis—Common.css changes can’t be previewed even locally). Common.css is not designed for styling templates, for example there are no good tools to associate its styles even for humans with the template it styles, let alone machine-readably; it’s loaded on all pages and thus edits to it have considerable performance impact, especially if it’s loaded on all pages of every single Wikimedia wiki; it can style anything on the page, including outside of the content area, so it’s too powerful to be edited by anyone; and so on. Styles that currently reside in Common.css should be migrated to TemplateStyles when moving to the global repo.