I've tried to add styles to the Confluence editor. It wouldn't take them. Perhaps one of Atlassian's values is "Don't f@@k the editor".

<web-resource name="editor-scripts" key="editor-scripts">

  <resource name="editor-templates.js" type="download"

    location="assets/js/editor-templates.js" />

  <!-- // TODO: Check whether I need a context here -->

</web-resource>

<web-resource name="editor-styles" key="editor-styles">

  <resource name="css/confluence-editor.css" type="download"

    location="assets/css/confluence-editor.css"/>
  <dependency>com.atlassian.confluence.plugins:confluence-editor-plugins</dependency>

  <dependency>${project.groupId}.${project.artifactId}:editor-scripts</dependency>

  <dependency>com.atlassian.auiplugin:ajs</dependency>

  <context>editor</context>

  <!-- // TODO: Check whether I need all the dependencies here -->
</web-resource>

My confluence-editor.css styles just wouldn't be accepted. Javascript injection worked just fine though. Perhaps I could use JS to inject my styles?

Yup. Adding this to my editor-templates.js file worked a treat:

var editorCss = AJS.params.staticResourceUrlPrefix

  + '/download/resources/me.davidsimpson.confluence.addon.awesome-addon'

  + ':editor-styles/css/confluence-editor.css';

if (AJS.params.isDevMode) {

  editorCss = editorCss + '?_=' + Date.now();

}
$('#wysiwygTextarea_ifr').contents().find('head').append(

  '<link type="text/css" rel="stylesheet" href="' + editorCss + '" media="all">'

);

Note the cache busting timestamp for local development.