diff --git a/Justfile b/Justfile
index fd4fd48..8049bd3 100644
--- a/Justfile
+++ b/Justfile
@@ -47,9 +47,20 @@ db-migrate:
 db-seed:
     docker exec -i colectivo-dev-db-1 psql -U postgres < supabase/seed.sql
 
-# Regenerate TypeScript types from Supabase schema
+# Regenerate TypeScript types from Supabase schema.
+# Writes to a tempfile first so a missing/failing `supabase` CLI does not
+# truncate the existing hand-curated database.ts (which would silently
+# break every TS consumer in the monorepo).
 db-types:
-    supabase gen types typescript --local > packages/types/src/database.ts
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if ! command -v supabase >/dev/null 2>&1; then
+        echo "supabase CLI not installed — database.ts is hand-curated, edit it manually" >&2
+        exit 1
+    fi
+    tmp=$(mktemp)
+    supabase gen types typescript --local > "$tmp"
+    mv "$tmp" packages/types/src/database.ts
     echo "Types written to packages/types/src/database.ts"
 
 # Open Supabase Studio
diff --git a/packages/types/src/database.ts b/packages/types/src/database.ts
index 5bc1cfc..b158f93 100644
--- a/packages/types/src/database.ts
+++ b/packages/types/src/database.ts
@@ -9,6 +9,7 @@ export type AvatarTypeEnum = 'initials' | 'emoji' | 'upload';
 export type MemberRoleEnum = 'admin' | 'member' | 'guest';
 export type ListStatusEnum = 'active' | 'completed' | 'archived';
 export type NoteColor = 'yellow' | 'green' | 'blue' | 'pink' | 'purple' | 'orange' | 'gray' | 'red';
+export type ThemeMode = 'light' | 'dark' | 'system';
 
 export interface Database {
 	public: {
@@ -22,6 +23,7 @@ export interface Database {
 					avatar_type: AvatarTypeEnum;
 					avatar_emoji: string | null;
 					avatar_url: string | null;
+					theme: ThemeMode;
 					created_at: string;
 					updated_at: string;
 				};
@@ -33,6 +35,7 @@ export interface Database {
 					avatar_type?: AvatarTypeEnum;
 					avatar_emoji?: string | null;
 					avatar_url?: string | null;
+					theme?: ThemeMode;
 					created_at?: string;
 					updated_at?: string;
 				};
@@ -44,6 +47,7 @@ export interface Database {
 					avatar_type?: AvatarTypeEnum;
 					avatar_emoji?: string | null;
 					avatar_url?: string | null;
+					theme?: ThemeMode;
 					created_at?: string;
 					updated_at?: string;
 				};
diff --git a/supabase/migrations/015_user_theme.sql b/supabase/migrations/015_user_theme.sql
new file mode 100644
index 0000000..50eebd9
--- /dev/null
+++ b/supabase/migrations/015_user_theme.sql
@@ -0,0 +1,16 @@
+-- Migration 015: theme preference on public.users (Fase 9.1)
+--
+-- Stores the user's chosen UI theme: 'light' | 'dark' | 'system'.
+-- New users (and existing rows on upgrade) default to 'system' so the UI
+-- inherits the OS preference until they explicitly opt in.
+--
+-- The JWT minted by GoTrue caches `auth.users` (not `public.users`), so
+-- adding this column does NOT invalidate existing sessions — clients read
+-- the value via PostgREST `SELECT theme FROM users WHERE id = auth.uid()`.
+
+ALTER TABLE public.users
+    ADD COLUMN theme text NOT NULL DEFAULT 'system'
+    CHECK (theme IN ('light', 'dark', 'system'));
+
+COMMENT ON COLUMN public.users.theme IS
+    'UI theme preference. ''system'' follows prefers-color-scheme.';
diff --git a/supabase/tests/009_user_theme.sql b/supabase/tests/009_user_theme.sql
new file mode 100644
index 0000000..b50869f
--- /dev/null
+++ b/supabase/tests/009_user_theme.sql
@@ -0,0 +1,69 @@
+-- pgTAP: public.users.theme column (migration 015)
+-- Run with: psql -U postgres -d postgres -f supabase/tests/009_user_theme.sql
+--
+-- Fase 9.1 — dark mode feature flagship. Adds `theme` to public.users with
+-- three valid values: 'light' | 'dark' | 'system'. New users default to
+-- 'system' so the UI matches the OS preference until they opt in.
+
+CREATE EXTENSION IF NOT EXISTS pgtap;
+
+BEGIN;
+SELECT plan(8);
+
+-- ── Column existence + type ─────────────────────────────────────────────
+SELECT has_column(
+    'public', 'users', 'theme',
+    'public.users has a theme column'
+);
+
+SELECT col_type_is(
+    'public', 'users', 'theme', 'text',
+    'public.users.theme is of type text'
+);
+
+SELECT col_not_null(
+    'public', 'users', 'theme',
+    'public.users.theme is NOT NULL'
+);
+
+SELECT col_default_is(
+    'public', 'users', 'theme', 'system',
+    'public.users.theme defaults to system'
+);
+
+-- ── Check constraint behaviour ──────────────────────────────────────────
+-- Use Ana (seed admin) so the row exists.
+UPDATE public.users SET theme = 'light'
+WHERE id = '11111111-1111-1111-1111-111111111111';
+SELECT is(
+    (SELECT theme FROM public.users WHERE id = '11111111-1111-1111-1111-111111111111'),
+    'light',
+    'theme can be set to light'
+);
+
+UPDATE public.users SET theme = 'dark'
+WHERE id = '11111111-1111-1111-1111-111111111111';
+SELECT is(
+    (SELECT theme FROM public.users WHERE id = '11111111-1111-1111-1111-111111111111'),
+    'dark',
+    'theme can be set to dark'
+);
+
+UPDATE public.users SET theme = 'system'
+WHERE id = '11111111-1111-1111-1111-111111111111';
+SELECT is(
+    (SELECT theme FROM public.users WHERE id = '11111111-1111-1111-1111-111111111111'),
+    'system',
+    'theme can be set to system'
+);
+
+-- Invalid value must be rejected by the check constraint.
+SELECT throws_ok(
+    $$UPDATE public.users SET theme = 'sepia' WHERE id = '11111111-1111-1111-1111-111111111111'$$,
+    '23514',
+    NULL,
+    'theme rejects values other than light|dark|system'
+);
+
+SELECT * FROM finish();
+ROLLBACK;