[MichaelBelousov]

An app built on fizzy could not ship its own themes or choose which one a fresh install starts on: the theme set was fizzy's alone (Editor registering Fizzy Dark / Light / Strawberry), and Settings.default_theme was the literal "Fizzy Dark". Graphl's port ran into exactly this (fizzyedit/graphl's report item 11) — it carries its own Graphl Dark / Light.

Two ways for an app to contribute, following -Dapp-layout:

At runtime, from a plugin (before the first frame):

• Host.registerTheme(theme) — the editor appends the theme beside the user's and the app's, replacing an entry of the same name in place (a reloaded plugin updates its theme instead of duplicating it). The host keeps its own copy of the name and tracks the registering plugin, the way demos do; the editor pulls them in postInit — the editor's own theme apply ran in init, before any plugin existed — then re-applies font sizes and the settings theme, so a registered theme lands shaped and sized like the built-ins. Plugin reloads and web-plugin loads pull again.
• Host.setDefaultTheme(name) — what a fresh settings store starts on. The stored setting now seeds empty ("the app decides") instead of seeding the comptime default, so a runtime default actually takes effect: the stored name is only written when the user picks, and the fallback chain at apply time is the stored name, then the runtime default, then Settings.default_theme.

At build time, for an app without plugins:

• -Dapp-theme — a LazyPath to a file with pub fn add(gpa, themes: *std.ArrayList(dvui.Theme)) !void; the editor appends its themes beside the user's. The module wires into the executable's root, the web build, and the test module, each only when the build was given it (the editor's has_app_theme branch is comptime-off otherwise, so the import is never analyzed).
• -Dapp-default-theme — the comptime fallback Settings.default_theme uses when no runtime default was set.

Fizzy's own build passes neither and behaves exactly as before. Verified with a smoke-test themes file and -Dapp-default-theme on the native and web builds, the full suite (with a new Host.registerTheme test mirroring registerDemo's), fmt, and lint.

[MichaelBelousov]

hmmm I actually wanted to do this at runtime... will try again

EDIT: done