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>

main/TopBar.tscn

@@ -17,6 +17,7 @@ script = ExtResource("top_bar_script")
 layout_mode = 0
 offset_right = 1920.0
 offset_bottom = 56.0
+theme_type_variation = &"TopBarBackground"
 
 [node name="Title" type="Label" parent="." unique_id=1187907861]
 layout_mode = 0