Add a stylesheet to admin
Use a small active module or site template override.
Place your stylesheet at priv/lib/css/garden-admin.css in the site or an active module. Add priv/templates/_html_head_admin.tpl:
{% lib "css/garden-admin.css" %}
The admin head collects these templates from active applications. Keep selectors scoped to your own widget or a deliberate admin class; broad rules for every input or button can break other modules.
For a reusable module, use an OTP application such as zotonic_mod_garden_admin, with src/zotonic_mod_garden_admin.app.src and main module mod_garden_admin.erl. The application and module names differ intentionally. Follow the module creation task for its complete application metadata and activation.
Compile, enable the module if applicable, reload admin and inspect the stylesheet request. Test narrow and wide screens and the form's error state.