feat: move color themes into a bundled themes plugin

packages/plugin-sdk/cli/commands/build.mjs

@@ -5,7 +5,12 @@ import { fileURLToPath } from "node:url";
 import * as esbuild from "esbuild";
 import { BACKEND_EXTERNALS, FRONTEND_EXTERNALS } from "../lib/externals.mjs";
 import { staticUrlImports } from "../lib/static-url-imports.mjs";
-import { readManifest, resolveEntry, copyDir } from "../lib/plugin-dir.mjs";
+import {
+  readManifest,
+  resolveEntry,
+  copyDir,
+  isThemeOnly,
+} from "../lib/plugin-dir.mjs";
 import { openapi } from "./openapi.mjs";
 import { splitByCoreClasses } from "../lib/css-classes.mjs";
 
@@ -232,32 +237,41 @@ export async function build({ cwd }) {
   fs.mkdirSync(outDir, { recursive: true });
 
   const backendEntry = resolveEntry(cwd, BACKEND_ENTRIES);
-  if (!backendEntry) {
+  if (!backendEntry && !isThemeOnly(manifest)) {
     throw new Error(
       `${pluginId}: no backend entry (looked for ${BACKEND_ENTRIES.join(", ")})`,
     );
   }
 
-  await esbuild.build({
-    entryPoints: [backendEntry],
-    outfile: path.join(outDir, "backend.js"),
-    bundle: true,
-    format: "esm",
-    platform: "node",
-    target: "node22",
-    sourcemap: true,
-    logLevel: "warning",
-    // A native dependency's compiled .node binary is resolved by the
-    // package's own relative paths, which break once esbuild inlines its JS
-    // elsewhere. Declaring it in nativeDependencies keeps it a real
-    // node_modules import instead, resolved at runtime like a host-provided
-    // package.
-    external: [...BACKEND_EXTERNALS, ...(manifest.nativeDependencies ?? [])],
-    loader: BINARY_LOADERS,
-    // Bundled CJS deps still call require() for Node builtins at runtime.
-    // ESM has no ambient require, so give esbuild's require shim a real one.
-    banner: { js: REQUIRE_BANNER },
-  });
+  if (backendEntry) {
+    await esbuild.build({
+      entryPoints: [backendEntry],
+      outfile: path.join(outDir, "backend.js"),
+      bundle: true,
+      format: "esm",
+      platform: "node",
+      target: "node22",
+      sourcemap: true,
+      logLevel: "warning",
+      // A native dependency's compiled .node binary is resolved by the
+      // package's own relative paths, which break once esbuild inlines its JS
+      // elsewhere. Declaring it in nativeDependencies keeps it a real
+      // node_modules import instead, resolved at runtime like a host-provided
+      // package.
+      external: [...BACKEND_EXTERNALS, ...(manifest.nativeDependencies ?? [])],
+      loader: BINARY_LOADERS,
+      // Bundled CJS deps still call require() for Node builtins at runtime.
+      // ESM has no ambient require, so give esbuild's require shim a real one.
+      banner: { js: REQUIRE_BANNER },
+    });
+  } else {
+    // Core loads every plugin through a backend entry, so a theme only
+    // plugin gets an empty one.
+    fs.writeFileSync(
+      path.join(outDir, "backend.js"),
+      "export function activate() {}\n",
+    );
+  }
 
   for (const worker of readWorkerEntries(cwd, pluginId)) {
     await esbuild.build({

packages/plugin-sdk/cli/commands/validate.mjs

@@ -1,7 +1,7 @@
 import fs from "node:fs";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
-import { readManifest } from "../lib/plugin-dir.mjs";
+import { readManifest, isThemeOnly } from "../lib/plugin-dir.mjs";
 
 /**
  * The manifest rules live in src/manifest.ts, which the server uses too, so
@@ -25,7 +25,8 @@ export async function validate({ cwd }) {
   // The manifest names files. They have to be there.
   const referenced = [
     raw.backend ?? "dist/backend.js",
-    raw.frontend ?? "dist/frontend.js",
+    // A theme only plugin has no frontend code.
+    ...(isThemeOnly(raw) ? [] : [raw.frontend ?? "dist/frontend.js"]),
     raw.locales ?? "locales",
   ];
   for (const rel of referenced) {
@@ -35,6 +36,7 @@ export async function validate({ cwd }) {
   }
 
   problems.push(...validateThemeFiles(cwd, raw, validateThemeCss));
+  problems.push(...validateLookCapabilities(cwd, raw));
   problems.push(
     ...(await validateMigrations(cwd, raw.id ?? path.basename(cwd))),
   );
@@ -78,6 +80,39 @@ export function validateThemeFiles(cwd, raw, validateThemeCss) {
   return problems;
 }
 
+/**
+ * ui:layout and ui:theme must match what the manifest contributes, both
+ * ways. The manifest check already flags a contribution without its
+ * capability; this flags a capability with nothing behind it, and a layout
+ * with no frontend to register it.
+ */
+export function validateLookCapabilities(cwd, raw) {
+  const problems = [];
+  const capabilities = Array.isArray(raw?.capabilities) ? raw.capabilities : [];
+  const layouts = raw?.contributes?.layouts;
+  const themes = raw?.contributes?.themes;
+  const hasLayouts = Array.isArray(layouts) && layouts.length > 0;
+  const hasThemes = Array.isArray(themes) && themes.length > 0;
+  if (capabilities.includes("ui:layout") && !hasLayouts) {
+    problems.push('"ui:layout" is declared but contributes.layouts is empty');
+  }
+  if (capabilities.includes("ui:theme") && !hasThemes) {
+    problems.push('"ui:theme" is declared but contributes.themes is empty');
+  }
+  if (hasLayouts) {
+    const entries = ["index.tsx", "index.ts", "index.mjs", "index.js"];
+    const frontend = entries.some((entry) =>
+      fs.existsSync(path.join(cwd, "src", "frontend", entry)),
+    );
+    if (!frontend && raw.frontend === undefined) {
+      problems.push(
+        "contributes.layouts needs a frontend that calls app.registerLayout",
+      );
+    }
+  }
+  return problems;
+}
+
 /** Env vars core sets for everyone, so a plugin need not list them. */
 const CORE_ENV = new Set([
   "DATA_DIR",

packages/plugin-sdk/cli/lib/plugin-dir.mjs

@@ -14,6 +14,17 @@ export function readManifest(cwd) {
 }
 
 /** The first of these that exists, or null. */
+/** Only themes: CSS files and no code, so no source entry is needed. */
+export function isThemeOnly(manifest) {
+  const capabilities = manifest.capabilities ?? [];
+  return (
+    capabilities.length > 0 &&
+    capabilities.every((capability) => capability === "ui:theme") &&
+    Array.isArray(manifest.contributes?.themes) &&
+    manifest.contributes.themes.length > 0
+  );
+}
+
 export function resolveEntry(cwd, candidates) {
   for (const candidate of candidates) {
     const full = path.join(cwd, candidate);