diff --git a/agent/BACKLOG.md b/agent/BACKLOG.md
index 3a76e5f..c3cfb60 100644
--- a/agent/BACKLOG.md
+++ b/agent/BACKLOG.md
@@ -28,6 +28,23 @@ in [`docs/ROADMAP.md`](../docs/ROADMAP.md); map in
 
 ## NEXT
 
+### 79. Auto time-of-day theme pair (`VISION § D`) `[big]` — split
+Auto light/dark theme switch through the **existing** engine (a timer runs
+`nomarchy-theme-sync apply`), one pipeline. State: `settings.autoTheme`
+`{enable, day, night, sunrise, sunset}` in theme-state.json. Slices:
+1. ~~**CLI primitive + schema**~~ ✓ shipped 2026-07-10 —
+   `nomarchy-theme-sync auto` resolves day/night from
+   `settings.autoTheme.{enable,day,night,sunrise,sunset}` and applies (only
+   if different — idempotent, so the timer won't needlessly rebuild);
+   `--which` prints the decision, `--force`/`--no-switch` too. `cmd_apply`
+   core factored into `apply_named` (behavior unchanged). V1 + functional
+   tests (day/night/disabled/missing-slugs, apply writes state, idempotent).
+2. **Timer/hook** — systemd user timers at sunrise+sunset running
+   `nomarchy-theme-sync auto`, gated on `settings.autoTheme.enable`; apply
+   the correct theme at session start too. V2 (runNixOSTest).
+3. **Menu** — Look & Feel › Auto theme: enable/disable, pick day/night
+   themes, set times. Writes state.
+
 ### 76. Hibernation default — all agent work shipped `[blocked:hw]`
 Hibernation + zram on by default (product intent, Bernardo 2026-07-10).
 **All V0/V1/V2 slices done** (2026-07-10); only the V3 check remains — on a
diff --git a/agent/JOURNAL.md b/agent/JOURNAL.md
index 4478f08..f9f3b78 100644
--- a/agent/JOURNAL.md
+++ b/agent/JOURNAL.md
@@ -17,6 +17,25 @@ Template:
 
 ---
 
+## 2026-07-10 — #79 slice 1: auto time-of-day theme CLI primitive
+- **Task:** Split BACKLOG #79 (`VISION § D`, `[big]`) into 3 slices (CLI /
+  timer / menu); took slice 1.
+- **Did:** `nomarchy-theme-sync auto` — reads
+  `settings.autoTheme.{enable,day,night,sunrise,sunset}`, computes day/night
+  by the local clock, and applies the matching preset **only if different**
+  (idempotent → the slice-2 timer won't rebuild needlessly). `--which`
+  prints the decision without switching; `--force`/`--no-switch` too.
+  Factored `cmd_apply`'s core into `apply_named` so `auto` reuses the exact
+  same one engine (GOALS: no second pipeline).
+- **Verified:** **V1** package builds; functional tests on the built bin
+  against scratch state: wide-day→day slug, 00:00–00:01 window→night,
+  disabled→no-op, missing day/night→die, `auto --no-switch` writes state,
+  re-run→"already on" no-op, plain `apply` still writes (refactor intact).
+  **V0** py_compile + flake check + `checks.theme-sync-validate` green.
+- **Pending:** slices 2 (systemd sunrise/sunset timers + login apply, V2)
+  and 3 (Look & Feel menu) remain under #79.
+- **Next suggestion:** #79 slice 2 — the timer/hook (V2 runNixOSTest).
+
 ## 2026-07-10 — #78 Omarchy migrant doc (docs/OMARCHY.md)
 - **Task:** BACKLOG #78 (promoted from PROPOSED) — Omarchy→Nomarchy
   one-pager (`VISION § F`).
diff --git a/pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py b/pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py
index 15e1723..9c6b490 100644
--- a/pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py
+++ b/pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py
@@ -18,6 +18,7 @@ Commands:
   apply <name|file.json>     merge a preset into the state + rebuild
   set <dotted.path> <value>  tweak one key (e.g. `set ui.gapsOut 16`) + rebuild
   get [dotted.path]          print the current state (or one key)
+  auto [--which]             apply the day/night theme for the current time
   validate                   check the state file against the schema (read-only)
   wallpaper                  apply the current wallpaper via swww
   bg [next|auto]             cycle the theme's wallpapers (instant, no rebuild)
@@ -466,11 +467,13 @@ def cmd_list(_args) -> None:
     print("\n".join(names))
 
 
-def cmd_apply(args) -> None:
-    candidate = Path(args.theme).expanduser()
-    preset_path = candidate if candidate.is_file() else find_preset(args.theme)
+def apply_named(theme: str, no_switch: bool = False) -> None:
+    """Merge a preset (name or JSON file) into the state and rebuild. Shared
+    by `apply` and `auto` so the day/night switch is the *same* one engine."""
+    candidate = Path(theme).expanduser()
+    preset_path = candidate if candidate.is_file() else find_preset(theme)
     if preset_path is None:
-        die(f"unknown theme '{args.theme}' (try `nomarchy-theme-sync list`)")
+        die(f"unknown theme '{theme}' (try `nomarchy-theme-sync list`)")
 
     preset = json.loads(preset_path.read_text())
     # Merge over current state. Presets carry a full appearance block —
@@ -483,14 +486,63 @@ def cmd_apply(args) -> None:
     state = deep_merge(old, preset)
     write_state(state)
     if auto_commit_enabled(old) or auto_commit_enabled(state):
-        auto_commit(f"nomarchy: apply theme {state.get('name', args.theme)}")
-    log(f"theme: {state.get('name', args.theme)}")
+        auto_commit(f"nomarchy: apply theme {state.get('name', theme)}")
+    log(f"theme: {state.get('name', theme)}")
     check_fonts(state)
-    if not args.no_switch:
+    if not no_switch:
         run_switch()
         apply_wallpaper(state)
 
 
+def cmd_apply(args) -> None:
+    apply_named(args.theme, no_switch=args.no_switch)
+
+
+def _hhmm_to_min(s: str, default: str) -> int:
+    """'HH:MM' → minutes since midnight; falls back to `default` on garbage."""
+    for candidate in (s, default):
+        try:
+            h, m = str(candidate).split(":")
+            return int(h) * 60 + int(m)
+        except (ValueError, AttributeError):
+            continue
+    return 0
+
+
+def cmd_auto(args) -> None:
+    """Apply the day or night theme for the current local time, per
+    settings.autoTheme = {enable, day, night, sunrise, sunset}. Slice 1 of
+    the auto time-of-day pair (BACKLOG #79): the timer (slice 2) and menu
+    (slice 3) drive this; it can also be run by hand or from a user cron."""
+    state = load_state()
+    at = (state.get("settings") or {}).get("autoTheme") or {}
+    if not at.get("enable"):
+        if not args.which:
+            log("auto-theme: off (enable via settings.autoTheme.enable)")
+        return
+    day, night = at.get("day"), at.get("night")
+    if not day or not night:
+        die("auto-theme needs settings.autoTheme.day and .night (theme slugs)")
+
+    sunrise = _hhmm_to_min(at.get("sunrise", "07:00"), "07:00")
+    sunset = _hhmm_to_min(at.get("sunset", "20:00"), "20:00")
+    now = time.localtime()
+    mins = now.tm_hour * 60 + now.tm_min
+    # Normal case sunrise < sunset: daytime is the span between them. If a
+    # user inverts them, treat the between-span as night (still coherent).
+    between = sunrise <= mins < sunset
+    daytime = between if sunrise <= sunset else not between
+    target = day if daytime else night
+
+    if args.which:
+        print(target)
+        return
+    if state.get("slug") == target and not args.force:
+        log(f"auto-theme: already on {target} ({'day' if daytime else 'night'})")
+        return
+    apply_named(target, no_switch=args.no_switch)
+
+
 def cmd_set(args) -> None:
     state = load_state()
     try:
@@ -590,6 +642,12 @@ def main() -> None:
     p.add_argument("path", nargs="?")
     p.set_defaults(func=cmd_get)
 
+    p = sub.add_parser("auto", help="apply the day/night theme for the current time (settings.autoTheme)")
+    p.add_argument("--which", action="store_true", help="print the theme that WOULD apply, don't switch")
+    p.add_argument("--force", action="store_true", help="apply even if already on the target theme")
+    p.add_argument("--no-switch", action="store_true", help="write state only, skip the rebuild")
+    p.set_defaults(func=cmd_auto)
+
     sub.add_parser("validate", help="check theme-state.json against the schema (read-only)").set_defaults(func=cmd_validate)
 
     sub.add_parser("wallpaper", help="apply the current wallpaper via swww").set_defaults(func=cmd_wallpaper)