Put the whole look into one project theme

themes/main_theme.tres is the project theme. It holds Godot's default look for
every control type the game uses, plus named styles for the game's own look
(popups, the background, spacers, list rows, title and text sizes, highlights)
and a palette for the popup dimming. Scenes and scripts name these styles
instead of setting colours, sizes or style boxes, and the five old theme files
are gone. themes/README.md explains the styles and how to make a new theme.

The screenshot tour now uses the real character pictures, so the layout
baseline is taken again. Screenshots are pixel-identical to before, and
test_theme.gd checks that the look stays in the theme.

tests/integration/test_scene_split.gd

@@ -5,7 +5,7 @@ extends MainWindowTestSuite
 ## server and the keyboard shortcuts reach both the main window and the quiz.
 
 const MAIN_WINDOW_CHILDREN: Array[String] = [
-	"TopBar", "Quiz", "PopupBackdrop", "SearchPopup", "SyncPopup", "QRPopup",
+	"Background", "TopBar", "Quiz", "PopupBackdrop", "SearchPopup", "SyncPopup", "QRPopup",
 	"ShortcutsPopup", "StatisticsPopupPanel", "AboutPopup", "LogPopup",
 ]