feat(settings): persist the theme through the settings binding

The palette lives in Lua, so C++ stores the name only and ui.setTheme()
writes through here before rebuilding and repainting.

native/src/bindings/core/settings.cpp

@@ -47,6 +47,19 @@ int setTimezone(lua_State* state) {
       state, Runtime::from(state)->settings().setTimezone({value, length}));
 }
 
+int getTheme(lua_State* state) {
+  const std::string theme = Runtime::from(state)->settings().theme();
+  lua_pushlstring(state, theme.data(), theme.size());
+  return 1;
+}
+
+int setTheme(lua_State* state) {
+  size_t length = 0;
+  const char* value = luaL_checklstring(state, 1, &length);
+  return pushStatus(state,
+                    Runtime::from(state)->settings().setTheme({value, length}));
+}
+
 const luaL_Reg FUNCTIONS[] = {
     // --- Returns the saved rotation in degrees clockwise.
     // @return integer
@@ -64,6 +77,16 @@ const luaL_Reg FUNCTIONS[] = {
     // @return true|nil ok
     // @return string|nil error
     {"setTimezone", setTimezone},
+    // --- Returns the saved palette name. Apps read ui.getTheme() instead; this
+    // --- is the stored value, which only ui.setTheme() knows how to apply.
+    // @return string
+    {"getTheme", getTheme},
+    // --- Persists a palette name without applying it. Call ui.setTheme(), which
+    // --- writes through here and then rebuilds the palette and repaints.
+    // @param theme string
+    // @return true|nil ok
+    // @return string|nil error
+    {"setTheme", setTheme},
     {nullptr, nullptr},
 };