For CSS, the equivalent in a plugin is [plugin]/assets/stylesheets/common/[file.scss]
Matching this format, I’ve tried adding a file at [plugin]/assets/views/layouts/common/header.html, but it didn’t work (just doing views/header.html didn’t work either).
I can change the templates, but for simple html changes like we do in themes a simple html change in header.html (or the equivalent) would be nice. For example, where would you put the following in a plugin (that’s already doing complex server side stuff, hence the need for a plugin):