Use Design → Custom CSS only after built-in theme controls cannot produce the required result. Keep selectors narrow, avoid hiding required fields or disclosures, and do not change behavior with CSS.

Test every product state, language, viewport, keyboard focus, error message, embed, and popup. Avoid selectors based on incidental generated structure that may change.

Record why the CSS exists and re-test after builder or theme updates. Remove unused rules rather than layering overrides indefinitely.

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