First product

app/theme.py

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+from dataclasses import dataclass
+from typing import Final
+
+
+@dataclass(frozen=True)
+class ThemeDefinition:
+    slug: str
+    label: str
+
+
+THEMES: Final[tuple[ThemeDefinition, ...]] = (
+    ThemeDefinition(slug="terracotta", label="Terracotta"),
+    ThemeDefinition(slug="fjord", label="Fjord"),
+    ThemeDefinition(slug="spruce", label="Spruce"),
+)
+_THEME_SLUGS: Final[frozenset[str]] = frozenset(theme.slug for theme in THEMES)
+DEFAULT_THEME: Final[str] = THEMES[0].slug
+
+
+def resolve_theme(candidate: str | None) -> str:
+    if candidate and candidate in _THEME_SLUGS:
+        return candidate
+    return DEFAULT_THEME
+