There are numerous approaches here, but consider, in order of preference:
- CSS
- Use of plugin outlets
- jQuery/equivalent native javascript edits of the DOM (not sure how easy this will be because this is not a component, so I don’t think it will be that easy to schedule manipulation as there are no event hooks).
- altering the template https://github.com/discourse/discourse/blob/master/app/assets/javascripts/discourse/app/templates/composer.hbs
The last option is risky, because this file will probably get a lot of updates over time (last update was 7 days ago!) and you don’t normally want to be overriding templates, especially those that frequently change, as you will mask stuff from core too much and break things.