default --source to the OS accent, and drop the all-literal case

A stylesheet where every role is literal is a static theme, which the theme page
already emits -- so the branch that dropped --u and --source for it was carrying a
case this file should not serve. Per-role literals stay: shadow, scrim and the fixed
2021 error hue are a third of all roles and cost nothing to emit exactly.

--source now defaults to AccentColor rather than a hardcoded purple, so an import
with no configuration already themes to the user. Verified in both engines that it
resolves as a relative-colour origin through a custom property with maths on the
channels -- CSS.supports() cannot answer that, since a value containing var() is
always accepted at parse time and only fails at computed-value time. AccentColor
also predates relative colour syntax in every engine, so anything that can parse
this file has it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: KTibow

scripts/generate-livetheme.ts

@@ -648,12 +648,10 @@ const hoistPass = (decls: string[]) => {
 
 // --- emit --------------------------------------------------------------------
 const usedKeys = new Set(entries.filter((e) => !e.literal).map((e) => e.key));
-const allLiteral = usedKeys.size === 0;
 const CE = hoistable("max(hypot(a,b),.00001)");
 const pd: string[] = [];
-if (!allLiteral)
-  // --u carries: L = source lightness, (a,b) = unit hue vector, alpha = source chroma
-  pd.push(`--u:oklab(from var(--source) l calc(a/${CE}) calc(b/${CE}) / calc(${CE}))`);
+// --u carries: L = source lightness, (a,b) = unit hue vector, alpha = source chroma
+pd.push(`--u:oklab(from var(--source) l calc(a/${CE}) calc(b/${CE}) / calc(${CE}))`);
 for (const [key, palette] of palInfo) {
   if (!usedKeys.has(key)) continue;
   let base = "var(--u)";
@@ -709,21 +707,23 @@ const rd = Object.entries(byRole).map(([rn, m]: [string, any]) => {
   return `--m3c-${rn}:${l === d ? l : `light-dark(${l},${d})`}`;
 });
 
-// Standalone on purpose: one @import and a --source is the whole API. The defaults
-// on :root make it render before anything is overridden, and .m3-theme lets a subtree
-// carry its own --source -- the custom properties are re-evaluated per element, so a
-// nested theme just works.
+// Standalone on purpose: one @import is the whole API. --source defaults to the OS
+// accent so an unconfigured import already themes to the user, and .m3-theme lets a
+// subtree carry its own --source -- the custom properties are re-evaluated per
+// element, so a nested theme just works.
 const css =
   `/* ${variant} -- Material colour roles computed live from --source, in CSS.\n` +
   ` * Generated by scripts/generate-livetheme.ts; do not edit.\n` +
   ` *\n` +
   ` *   @import "m3-svelte/etc/livetheme/${variant}.css";\n` +
+  ` *\n` +
+  ` * --source defaults to the OS accent colour. To pin one instead:\n` +
   ` *   :root { --source: #0b57d0; }\n` +
   ` *\n` +
-  ` * Any subtree can override it instead:\n` +
+  ` * Any subtree can override it:\n` +
   ` *   <div class="m3-theme" style="--source: #b33b15">\n` +
   ` */\n` +
-  `:root{${allLiteral ? "" : "--source:#6750a4;"}color-scheme:light dark}\n` +
+  `:root{--source:AccentColor;color-scheme:light dark}\n` +
   `:root,.m3-theme{\n${hoistPass([...pd, ...rd])}\n}\n`;
 // Alongside the library's other stylesheets, so svelte-package ships it and it is
 // imported the same way they are. scripts/generate-tailwind.ts writes into the same