Hi Joe, thanks for clarification.
some code snippets for kibana 5 plugin, which change CSS:
index.js
export default function (kibana) {
return new kibana.Plugin({
id: 'hugo_kibana_plugin',
require: ['kibana', 'elasticsearch'],
uiExports: {
  hacks: [
    'plugins/hugo_kibana_plugin/hack'
  ]
}
});
};
public/hack.js
import 'plugins/hugo_kibana_plugin/less/main.less';
public/less/main.less
.global-nav {
  background-color: yellow;
}