Quick answer
Generate or prepare the right landscape image for a custom Codex background without broken crops or unreadable text.
Use a 16:10 landscape master
The recommended master is 3200 × 2000. It matches common laptop displays and still gives enough room for 16:9 and 4:3 crops. A vertical phone wallpaper cannot be repaired by CSS without losing major content.
Design around the interface
Place the main subject toward the right third and keep the center-left calm. Do not bake labels, buttons, or chat windows into the image; they compete with the real interface and can mislead users.
Preview more than one window
A strong background works in a full desktop window, a normal task view, and a narrower window. Use focal coordinates and responsive overlays, but generate alternate crops when the composition is sensitive.
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.