Board Thread:Support Requests - Designing Your Wiki/@comment-34989956-20200821155847/@comment-9605025-20200821191446

You can't. It has to be via site CSS. What you can do is target specific ones. Depending on the specifics of the criteria for changing the width, you can use either the infobox's theme, theme-source, or name. You can also target based on the page it is on but then you would need to change the CSS if the page gets renamed. Please note that anything you do via site CSS will not be reflected on mobile view.