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/unit/test_settings_files.gd

@@ -6,7 +6,7 @@ const SAVED_VALUES: Dictionary = {
 	&"auto_repeat_song": false, &"stop_after_current": false, &"hide_next_track": false,
 	&"hide_beginning": true, &"hide_for_seconds": 7.0, &"hide_length": true,
 	&"hide_ticks": true, &"winning_score": 13, &"inspiration_list_speed": 4,
-	&"fullscreen": true, &"last_seen_version": "1.2.3",
+	&"fullscreen": true, &"last_seen_version": "1.2.3", &"theme_id": "original_web",
 }
 
 var _snapshot: AutoloadSnapshot