I don’t know how extensive the DOM changes are that you have in mind to do, or if this is for your own site or to share with others.
But if relatively trivial and for your own site maybe an Admin -> Customize would suffice. eg. the script part here:
https://meta.discourse.org/t/apply-css-to-all-custom-non-badge-user-titles/51376/36?u=mittineague