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/SettingsPopup.tscn

@@ -143,6 +143,21 @@ theme_type_variation = &"HighlightMedium"
 text = "No pictures found"
 text_overrun_behavior = 3
 
+[node name="ThemeRow" type="HBoxContainer" parent="SettingsPanel/VBoxContainer"]
+layout_mode = 2
+
+[node name="Title" type="Label" parent="SettingsPanel/VBoxContainer/ThemeRow"]
+layout_mode = 2
+size_flags_horizontal = 3
+theme_type_variation = &"SettingsRowLabel"
+text = "Theme"
+
+[node name="ThemeButton" type="OptionButton" parent="SettingsPanel/VBoxContainer/ThemeRow"]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(180, 0)
+layout_mode = 2
+focus_mode = 0
+
 [node name="FullscreenButton" type="CheckButton" parent="SettingsPanel/VBoxContainer" unique_id=400304684]
 unique_name_in_owner = true
 layout_mode = 2