Quick answer
Why theme is the clearest English term for Codex customization, and when skin or background is still useful.
Theme is the category word
In English product language, theme naturally covers a coordinated visual system: colors, surfaces, typography, and sometimes artwork. It is the clearest word for a collection users can browse, install, and share.
Background is one feature
Background accurately describes the image layer, but it does not describe the supporting palette, code surfaces, buttons, or installation behavior. Use it for image-specific guides and filters.
Skin is a bridge term
Skin is understood in gaming and desktop-customization communities, but can sound unofficial or cosmetic. Use it when referencing existing community projects, not as the primary product category.
Compatibility note: Native Codex appearance controls and advanced visual image layers are different. Get Codex Theme labels the delivery path and never treats an experimental visual layer as an official Appearance feature.