Default the site theme to the visitor's system preference

docs.decentraland.org sends every first-time visitor to light mode. GitBook supports a system default since March 2026 (`themes.default: "system"`).

This isn't in the repo — it's a GitBook dashboard setting: site → Customization → Modes → Default mode → **System** (keep the mode toggle on). Any space with its own Customization override (player/creator/contributor/apis and language variants) needs the same flip, or a reset to inherit the site default.

Verify afterwards: `curl -s https://docs.decentraland.org/ | grep -o '("class","theme","[^"]*"'` should report `"system"` instead of `"light"`. Note: visitors who previously used the footer toggle have a persisted explicit choice (localStorage key `theme`) that overrides any new default — expected behavior.

Open the GitBook dashboard for docs.decentraland.org and check Site → Customization → Modes, then review the listed player, creator, contributor, APIs, and language-variant spaces for overrides. Set each default mode to System or reset it to inherit, while keeping the mode toggle enabled. Verify with the provided curl and grep command that the reported theme is "system".

Written by the indexing model from the issue text.