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>

tools/take_screenshots.gd

@@ -3,6 +3,7 @@ extends SceneTree
 ##
 ## Usage:
 ##   MUSICPLAYER_CONFIG_DIR=$(mktemp -d) godot --path . -s tools/take_screenshots.gd -- <dir>
+##       [<theme id>]
 ##
 ## Setting [code]MUSICPLAYER_CONFIG_DIR[/code] keeps your saved quiz settings out of the
 ## pictures.
@@ -10,6 +11,9 @@ extends SceneTree
 ## Writes [code]<state>.png[/code] for each state and [code]layout.json[/code] with where
 ## every node is in each state. With [code]--headless[/code] only the layout is written.
 ## Compare two runs with [code]tools/compare_screenshots.py[/code].
+##
+## The pictures use the default theme, or the theme with the given id from
+## [code]Settings.THEMES[/code], e.g. [code]original_web[/code].
 
 const SIZE: Vector2i = Vector2i(1920, 1080)
 
@@ -37,6 +41,10 @@ func _take() -> void:
 	viewport.render_target_update_mode = SubViewport.UPDATE_ALWAYS
 	root.add_child(viewport)
 	var main: Control = load("res://main/MainWindow.tscn").instantiate()
+	if args.size() > 1:
+		# The game sets the root window's theme, which the viewport here is not under
+		var settings: Node = root.get_node("Settings")
+		main.theme = load(settings.theme_entry(args[1]).path)
 	viewport.add_child(main)
 
 	var layout: Dictionary = {}