Advanced theming addresses the most complex issues.
For example, participating in discussions on front end architecture, working on frameworks and tooling, and resolving technical issues that block other developers.
Here are are few areas in front end development where advanced themers are needed.
Bundlers
New community initiative: Frontend bundler
https://www.drupal.org/project/vite
Decoupled
https://www.drupal.org/project/vite_react_generator
https://github.com/pierrecdevs/react-drupal-decoupled
Importmaps
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script/type/importmap
https://www.drupal.org/project/importmaps
https://www.drupal.org/project/importmap
UI Suite
https://www.drupal.org/project/ui_patterns
https://www.drupal.org/project/ui_suite
https://www.drupal.org/project/ui_skins
https://www.drupal.org/project/ui_suite_daisyui
https://www.drupal.org/project/ui_styles
Design Systems
https://www.drupal.org/project/components