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/popup_backdrop.gd

@@ -27,5 +27,11 @@ static func add_to(parent: Control, popups: Array[BasePopup]) -> PopupBackdrop:
 	return backdrop
 
 
+func _notification(what: int) -> void:
+	# The dimming colour comes from the theme, which can be switched in Settings
+	if what == NOTIFICATION_THEME_CHANGED:
+		color = get_theme_color(&"backdrop", &"Palette")
+
+
 func _update() -> void:
 	visible = _popups.any(func(popup: BasePopup) -> bool: return popup.visible)