Add a theme picker and an Original web theme (#42)

Settings has a Theme row listing Settings.THEMES: Classic (main_theme.tres,
still the default) and Original web (original_web_theme.tres), which looks like
MusicFrontend: orange-bordered buttons, a #333 header and popups, #424242
panels, #4e4e4e alternating rows, a yellow playing song and a light grey
player bar. Picking one sets the root window's theme at once and saves it as
theme_id in settings.cfg, and it is applied at start.

The popup backdrop and score graph follow a theme switch. The top bar has its
own TopBarBackground style. test_theme.gd checks that every theme has
everything main_theme.tres has. The screenshot tool takes a theme id.

Closes #42

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_theme_picker.gd

@@ -0,0 +1,74 @@
+extends MainWindowTestSuite
+## Picking a theme in Settings switches the whole game's look at once and is saved.
+
+var _settings: SettingsPopup
+
+
+func before_test() -> void:
+	await super()
+	_settings = _window.get_node("SettingsPopup")
+
+
+func after_test() -> void:
+	Settings.apply_theme(Settings.THEMES[0].id)
+	await super()
+
+
+func _pick(theme_name: String) -> void:
+	var index: int = Settings.THEMES.map(
+			func(entry: Dictionary) -> String: return entry.name).find(theme_name)
+	_settings.theme_button.select(index)
+	_settings.theme_button.item_selected.emit(index)
+	await _runner.simulate_frames(1)
+
+
+func test_picker_lists_every_theme_and_shows_the_one_in_use() -> void:
+	var names: Array[String] = []
+	for index: int in _settings.theme_button.item_count:
+		names.append(_settings.theme_button.get_item_text(index))
+
+	assert_array(names).contains_exactly(["Classic", "Original web"])
+	assert_str(_settings.theme_button.get_item_text(_settings.theme_button.selected)) \
+			.is_equal("Classic")
+
+
+func test_picking_a_theme_switches_the_look_at_once() -> void:
+	var button: Button = _window.get_node("TopBar/%SyncButton")
+	var classic_text: Color = button.get_theme_color(&"font_color")
+	var panel: Panel = _window.get_node("SettingsPopup/SettingsPanel")
+	var popup_style: StyleBox = panel.get_theme_stylebox(&"panel")
+
+	await _pick("Original web")
+
+	assert_str(Settings.theme_id).is_equal("original_web")
+	assert_str(_window.get_tree().root.theme.resource_path) \
+			.is_equal("res://themes/original_web_theme.tres")
+	assert_object(panel.get_theme_stylebox(&"panel")).is_not_same(popup_style)
+	# The web version's buttons have orange text
+	assert_bool(button.get_theme_color(&"font_color") != classic_text).is_true()
+
+
+func test_chosen_theme_is_saved() -> void:
+	await _pick("Original web")
+
+	var saved: ConfigFile = ConfigFile.new()
+	assert_int(saved.load(Settings.saved_settings_path())).is_equal(OK)
+	assert_str(saved.get_value("app", "theme_id")).is_equal("original_web")
+
+
+func test_backdrop_and_graph_follow_the_theme() -> void:
+	var backdrop: PopupBackdrop = _window.get_node("PopupBackdrop")
+	var classic: Color = backdrop.color
+
+	await _pick("Original web")
+
+	assert_object(backdrop.color).is_equal(backdrop.get_theme_color(&"backdrop", &"Palette"))
+	assert_bool(backdrop.color != classic).is_true()
+
+
+func test_unknown_theme_falls_back_to_the_default() -> void:
+	Settings.apply_theme("no_such_theme")
+
+	assert_str(Settings.theme_id).is_equal("classic")
+	assert_str(_window.get_tree().root.theme.resource_path) \
+			.is_equal("res://themes/main_theme.tres")