solo1988 there is currently no general purpose layout editor for Flarum. You'll either need to use custom CSS (if applicable), your own extension (that can use a few lines in extend.php instead of a fully fledged extension) or reuse features from another extension to achieve your goals. For that last point I'm thinking of Flagrow Ads that can be used to inject HTML in a few places, and it doesn't have to be an actual advertisement.
One last option would be to manually inject content via javascript/jquery but I wouldn't recommend this unless you know the possible consequences of manually altering the Mithril templates, in which case you'd probably also know how to achieve the change via the proper Flarum extension method.
