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>

themes/main_theme.tres

@@ -1331,6 +1331,8 @@ TitleLarge/base_type = &"Label"
 TitleLarge/font_sizes/font_size = 50
 TitleMedium/base_type = &"Label"
 TitleMedium/font_sizes/font_size = 35
+TopBarBackground/base_type = &"PanelContainer"
+TopBarBackground/styles/panel = SubResource("StyleBoxFlat_wl3gy")
 TooltipLabel/colors/font_color = Color(0.875, 0.875, 0.875, 1)
 TooltipLabel/colors/font_outline_color = Color(0, 0, 0, 1)
 TooltipLabel/colors/font_shadow_color = Color(0, 0, 0, 0)