fix(code): re-apply theme preference on /reload (#4514)

`/reload` now picks up a theme default saved in another session and
switches to it.

---

`/reload` (in `libs/code`) rebuilt the theme *registry*
(`theme.reload_registry()` + `_register_custom_themes()`) but never
re-evaluated which theme should be *active* — it never re-ran
`_load_theme_preference()` or reassigned `self.theme`. So a per-terminal
default (`[ui.terminal_themes][TERM_PROGRAM]`) or global default saved
by another session was ignored until the app restarted. This re-resolves
the preference after the registry reload and applies it (with
terminal-background sync + CSS refresh), matching startup resolution
order (env → terminal default → `[ui].theme` → default). Re-applying the
on-disk preference is the intended `/reload` semantic, so it
deliberately overrides an unsaved in-session `/theme` choice.

Made by [Open
SWE](https://openswe.vercel.app/agents/7edbf211-076a-afd0-c0a8-76c44c422e00)

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

libs/code/deepagents_code/app.py

@@ -9642,6 +9642,28 @@ class DeepAgentsApp(App):
                 theme_reload_ok = False
                 logger.warning("Failed to reload user themes", exc_info=True)
 
+            # Re-resolve and apply the theme preference so a per-terminal or
+            # global default saved by another session is picked up. This
+            # re-syncs to on-disk config using the same resolution as startup
+            # (env -> [ui.terminal_themes][TERM_PROGRAM] -> [ui].theme ->
+            # default), which intentionally overrides an unsaved in-session
+            # `/theme` choice. Guarded on the registry reload succeeding since
+            # the target theme must be registered before it can be applied.
+            theme_switched_to: str | None = None
+            if theme_reload_ok:
+                try:
+                    new_theme = _load_theme_preference()
+                    if new_theme != self.theme and new_theme in theme.get_registry():
+                        self.theme = new_theme
+                        self.sync_terminal_background()
+                        self.refresh_css(animate=False)
+                        theme_switched_to = new_theme
+                except Exception:
+                    logger.warning(
+                        "Failed to re-apply theme preference on reload",
+                        exc_info=True,
+                    )
+
             # Re-discover skills so autocomplete reflects any new/removed
             # skills. Run via the same exclusive-group worker used at
             # startup so any in-flight startup discovery is cancelled
@@ -9667,6 +9689,10 @@ class DeepAgentsApp(App):
             report += "\nModel config caches cleared."
             if theme_reload_ok:
                 report += "\nTheme registry reloaded."
+                if theme_switched_to is not None:
+                    entry = theme.get_registry().get(theme_switched_to)
+                    label = entry.label if entry is not None else theme_switched_to
+                    report += f"\nSwitched theme to {label}."
             else:
                 report += (
                     "\nTheme registry reload failed. Check config.toml for errors."

libs/code/tests/unit_tests/test_reload.py

@@ -828,3 +828,88 @@ class TestReloadSkillReport:
         # Critical: must not claim every prior skill was removed.
         assert "Removed:" not in text
         assert "Skills updated" not in text
+
+
+class TestReloadThemeReapply:
+    """`/reload` should re-apply the resolved theme preference.
+
+    Guards the cross-session behavior: saving a per-terminal (or global)
+    default theme in one window should be picked up by an already-running
+    session's `/reload`, matching startup resolution.
+    """
+
+    async def _run_reload_theme(
+        self,
+        monkeypatch: pytest.MonkeyPatch,
+        *,
+        initial_theme: str,
+        resolved_theme: str,
+    ) -> tuple[str, str]:
+        """Drive `/reload` once with a stubbed preference resolver.
+
+        Args:
+            monkeypatch: pytest fixture for restorable patching.
+            initial_theme: theme active before reload.
+            resolved_theme: value `_load_theme_preference` returns on reload.
+
+        Returns:
+            The active theme after reload and the mounted `AppMessage` text.
+        """
+        from deepagents_code import app as app_module
+        from deepagents_code.app import DeepAgentsApp
+        from deepagents_code.widgets.messages import AppMessage
+
+        app = DeepAgentsApp()
+        async with app.run_test() as pilot:
+            await pilot.pause()
+            app.theme = initial_theme
+
+            async def _fake_discover() -> bool:  # noqa: RUF029  # awaited by handler
+                return True
+
+            monkeypatch.setattr(app, "_discover_skills", _fake_discover)
+            monkeypatch.setattr(
+                app_module, "_load_theme_preference", lambda: resolved_theme
+            )
+
+            await app._handle_command("/reload")
+            await pilot.pause()
+
+            text = "\n".join(str(w._content) for w in app.query(AppMessage))
+            return app.theme, text
+
+    async def test_switches_to_new_default(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """A newly resolved preference should become the active theme."""
+        active, text = await self._run_reload_theme(
+            monkeypatch,
+            initial_theme="langchain",
+            resolved_theme="langchain-light",
+        )
+        assert active == "langchain-light"
+        assert "Switched theme to" in text
+
+    async def test_no_switch_when_unchanged(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """When the resolved preference matches the active theme, no switch."""
+        active, text = await self._run_reload_theme(
+            monkeypatch,
+            initial_theme="langchain",
+            resolved_theme="langchain",
+        )
+        assert active == "langchain"
+        assert "Switched theme to" not in text
+
+    async def test_unregistered_preference_ignored(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """A resolved name that isn't registered must not change the theme."""
+        active, text = await self._run_reload_theme(
+            monkeypatch,
+            initial_theme="langchain",
+            resolved_theme="not-a-real-theme",
+        )
+        assert active == "langchain"
+        assert "Switched theme to" not in text