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>

popups/settings_popup.gd

@@ -1,9 +1,9 @@
 class_name SettingsPopup
 extends BasePopup
-## The application settings: which MusicServer or music folder to play from, fullscreen,
-## how many songs to download ahead, whether to prefer less played soundtracks and whether
-## plays count in the statistics, and a button that opens the folder the settings are
-## kept in. Opened from the Settings button in the top bar.
+## The application settings: which MusicServer or music folder to play from, the theme,
+## fullscreen, how many songs to download ahead, whether to prefer less played soundtracks
+## and whether plays count in the statistics, and a button that opens the folder the
+## settings are kept in. Opened from the Settings button in the top bar.
 ##
 ## A music folder (see [MusicFolder]) is used instead of the server until Use server is
 ## pressed or the game is closed. The settings that need the server are off meanwhile.
@@ -25,6 +25,7 @@ const PADDING: Vector2 = Vector2(20, 12)
 @onready var settings_panel: Panel = %SettingsPanel
 @onready var select_server_button: OptionButton = %SelectServerButton
 @onready var fullscreen_button: CheckButton = %FullscreenButton
+@onready var theme_button: OptionButton = %ThemeButton
 @onready var cache_label: Label = %CacheLabel
 @onready var lower_cache_button: Button = %LowerCacheButton
 @onready var increase_cache_button: Button = %IncreaseCacheButton
@@ -56,6 +57,10 @@ var _panel_middle_y: float = 0.0
 func _ready() -> void:
 	for server: String in Settings.servers:
 		select_server_button.add_item(server)
+	for entry: Dictionary in Settings.THEMES:
+		theme_button.add_item(entry.name)
+	theme_button.select(Settings.THEMES.find(Settings.theme_entry(Settings.theme_id)))
+	theme_button.item_selected.connect(select_theme)
 	fullscreen_button.button_pressed = Settings.fullscreen
 	select_server_button.select(Settings.selected_server)
 	cache_label.text = str(Playlist.number_of_tracks_to_preload)
@@ -83,6 +88,11 @@ func _ready() -> void:
 	_show_music_source()
 
 
+## Switches to the theme at [param index] in [constant Settings.THEMES] at once.
+func select_theme(index: int) -> void:
+	Settings.apply_theme(Settings.THEMES[index].id)
+
+
 func fullscreen() -> void:
 	Settings.set_fullscreen(!Settings.fullscreen)