feat(web): customizable placeholder, footer text, and color theme via BOR_* env vars

BOR_INPUT_PLACEHOLDER / BOR_FOOTER_TEXT / BOR_THEME (+ the indigo.css example theme); authoring guide: frontend/assets/themes/README.md, docs: README 'Customizing the look'.

tests/unit/test_config.py

@@ -397,3 +397,77 @@ def test_docs_branchs_garbage_ignored_when_repo_unset(
     s = _settings()
     assert s.docs_configured is False
     assert s.docs_branch == "bor docs.."  # stored verbatim, never used
+
+
+# --- UI customization (phase 62, TODO L3) ---
+
+
+def test_ui_customization_defaults_are_the_phase_61_copy() -> None:
+    """UNSET => byte-identical to the phase-61 neutral UI: the locked
+    phase-61 copy is the DEFAULT (composer placeholder + footer line),
+    and an empty theme = the built-in dark-tech palette."""
+    s = _settings()
+    assert s.input_placeholder == "Ask me anything…"
+    assert s.footer_text == "Powered by self-hosted models"
+    assert s.theme == ""
+
+
+def test_ui_customization_env_overrides(monkeypatch: pytest.MonkeyPatch) -> None:
+    """The three settings honor their ``BOR_`` env vars
+    (``BOR_INPUT_PLACEHOLDER`` / ``BOR_FOOTER_TEXT`` / ``BOR_THEME``);
+    placeholder/footer accept any string (empty is legal — the brand
+    layer then keeps the template default)."""
+    monkeypatch.setenv("BOR_INPUT_PLACEHOLDER", "Ask the vault…")
+    monkeypatch.setenv("BOR_FOOTER_TEXT", "Powered by my own models")
+    monkeypatch.setenv("BOR_THEME", "indigo.css")
+    s = _settings()
+    assert s.input_placeholder == "Ask the vault…"
+    assert s.footer_text == "Powered by my own models"
+    assert s.theme == "indigo.css"
+    monkeypatch.setenv("BOR_INPUT_PLACEHOLDER", "")
+    assert _settings().input_placeholder == ""  # empty stands
+
+
+def test_theme_validator_accepts_empty_and_bare_css_filename() -> None:
+    """Phase 62 (A5): empty = the built-in palette; a bare lowercase
+    ``.css`` filename (the ``indigo.css`` example) is the only
+    non-empty shape — dashes/underscores/digits are legal tokens."""
+    assert _settings().theme == ""  # "" passes
+    assert _settings(theme="indigo.css").theme == "indigo.css"
+    assert _settings(theme="dark-2026_v2.css").theme == "dark-2026_v2.css"
+
+
+@pytest.mark.parametrize(
+    ("bad", "match"),
+    [
+        # uppercase — the shape is lowercase-only
+        ("Indigo.css", "Indigo.css"),
+        # path escape — a theme is a filename, never a path
+        ("../evil.css", r"\.\./evil\.css"),
+        ("a/b.css", r"a/b\.css"),
+        ("/abs.css", r"got '/abs\.css'"),
+        # a missing extension is not a theme file
+        ("indigo", r"got 'indigo'"),  # must not match the example text
+    ],
+)
+def test_theme_validator_rejects_malformed_naming_the_value(
+    bad: str,
+    match: str,
+) -> None:
+    """A typo in ``BOR_THEME`` must kill startup, not silently 404 at
+    runtime — the rejection names the offending value (the phase-56
+    fail-loud house style) alongside the allowed shape."""
+    with pytest.raises(ValidationError, match=match):
+        _settings(theme=bad)
+
+
+def test_bor_theme_env_malformed_fails_startup_naming_value(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """The startup path: a malformed ``BOR_THEME`` in the environment
+    fails Settings construction loudly (the app builds its settings at
+    import time, so this is a refused boot), naming the value — the
+    E2E boots-check lands in task 05."""
+    monkeypatch.setenv("BOR_THEME", "../evil.css")
+    with pytest.raises(ValidationError, match=r"\.\./evil\.css"):
+        _settings()