Canonical source:
plugins_default/legacythemes/README.md- edit that file, not this page (this page is built live from it, nothing here is a tracked copy).
LegacyThemes
A CSS theme-switching framework, parallel to Legacy Settings - lets a player stack multiple named "style packs" (each just a CSS file plus some metadata) on top of the base theme, reorderable and removable, with the result persisted client-side.
Setup
Client-only. Its own client.js calls into LegacySettings.addTab/addOption directly to build its "Theming" tab - this dependency isn't declared in a dependencies.js (legacythemes has none), so nothing enforces that Legacy Settings is actually installed. It works today because Legacy Settings splices into the browser bundle at the beforebefore position while this plugin splices at before (see Client-Side Code) - earlier in the concatenated script, so its LegacySettings global already exists by the time this plugin's code runs. Removing Legacy Settings without also removing this plugin would break it at runtime, not at load time.
What it actually does
Other plugins register their own selectable theme by pushing onto LegacyThemesPlugin.stylePacks (an array, populated before game:permissionsAfterSetup finalizes the settings UI) - FancyGraphics is a real example. Three built-in packs ([Default Theme], "Old LegacyShell Assets", "Shell Shockers Assets") are always present alongside whatever other plugins add.
Selected packs' CSS is fetched and cached once (cacheAllStyles), then applied in reverse list order (later-added packs override earlier ones) by appending <style> elements to <head>. After applying, it reads back a fixed set of --customizer-* CSS custom properties to update the actual 3D scene's sky/ground colors (Customizer.skyColor/diffuseColor) - meaning a style pack can affect more than just UI chrome, it can genuinely retint the game world, by defining the right CSS variables.
Notes
--replaceImgSrc-<elementId> is a second, more specific CSS-variable convention this plugin looks for: any custom property named that way gets used to swap the src of the DOM element with that id - a way for a theme's CSS file to replace specific images (e.g. the logo) without any JS of its own.
This page was drafted with AI assistance and reviewed for accuracy. If something looks wrong, please open a PR or flag it.