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>
