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>

data/settings.gd

@@ -3,6 +3,9 @@ extends Node
 ##
 ## Registered as the [code]Settings[/code] autoload.
 
+## Emitted when another theme is used, for code that reads theme colours itself.
+signal theme_changed
+
 const MAX_NUMBER_OF_PLAYERS: int = 7
 ## How many letters a search needs before it also finds names with the letters in order
 ## but apart, e.g. "lttp" for "A Link to the Past". Shorter searches only find names
@@ -26,8 +29,14 @@ const SAVED_SETTINGS: Dictionary[String, Array] = {
 		&"inspiration_list_speed",
 	],
 	"window": [&"fullscreen"],
-	"app": [&"last_seen_version"],
+	"app": [&"last_seen_version", &"theme_id"],
 }
+## The looks the game can have, picked in Settings: an id that is saved, the name shown
+## and the theme file. The first is the default. See themes/README.md.
+const THEMES: Array[Dictionary] = [
+	{"id": "classic", "name": "Classic", "path": "res://themes/main_theme.tres"},
+	{"id": "original_web", "name": "Original web", "path": "res://themes/original_web_theme.tres"},
+]
 ## When this environment variable is set, the settings files are kept in that folder
 ## instead of the user's config folder. The tests set it so they never touch the real files.
 const CONFIG_DIR_ENV: String = "MUSICPLAYER_CONFIG_DIR"
@@ -85,6 +94,8 @@ var queue_ids: Dictionary[String, String] = {}
 ## The version the game last showed the About popup for. When it is not [member version],
 ## this version is new and the main window opens the About popup to show what is new.
 var last_seen_version: String = ""
+## The id of the theme in use, one of [constant THEMES].
+var theme_id: String = THEMES[0].id
 ## The folder songs are played from instead of the server, picked in the settings, or null
 ## to use the server. Kept for this session only.
 var music_folder: MusicFolder = null
@@ -103,6 +114,7 @@ var version: String = ProjectSettings.get_setting("application/config/version",
 func _ready() -> void:
 	apply_config(load_config())
 	load_saved_settings()
+	_use_theme(theme_id)
 	if fullscreen:
 		DisplayServer.window_set_mode(DisplayServer.WINDOW_MODE_FULLSCREEN)
 
@@ -246,6 +258,28 @@ func characters_dir() -> String:
 	return ""
 
 
+## Switches the whole game to the theme [param id] from [constant THEMES] and saves the
+## choice. Controls without a theme of their own take it from the root window.
+func apply_theme(id: String) -> void:
+	_use_theme(id)
+	save_settings()
+
+
+## The theme in [constant THEMES] with [param id], or the default one if there is none.
+func theme_entry(id: String) -> Dictionary:
+	for entry: Dictionary in THEMES:
+		if entry.id == id:
+			return entry
+	return THEMES[0]
+
+
+func _use_theme(id: String) -> void:
+	var entry: Dictionary = theme_entry(id)
+	theme_id = entry.id
+	get_tree().root.theme = load(entry.path)
+	theme_changed.emit()
+
+
 func delete_children(node: Node) -> void:
 	for n: Node in node.get_children():
 		node.remove_child(n)