fix(css): scope the subject compound so theme-dark rules keep matching

tests/scope-css.test.ts

@@ -46,6 +46,16 @@ describe("scoping the built stylesheet", () => {
     );
   });
 
+  it("scopes the subject, so an ancestor outside the root still matches", () => {
+    expect(one(".theme-dark .hue-chip{c:1}")).toBe(
+      ".theme-dark :is(.bcal-root, .bcal-root *).hue-chip { c: 1; }",
+    );
+    expect(one(".a > button{c:2}")).toBe(
+      ".a > button:is(.bcal-root, .bcal-root *) { c: 2; }",
+    );
+    expect(one(".a *{c:3}")).toBe(".a :is(.bcal-root, .bcal-root *) { c: 3; }");
+  });
+
   it("leaves rules that already name the root alone", () => {
     const reset = ".bcal-root.bcal-root button{cursor:pointer}";
     expect(one(reset)).toBe(".bcal-root.bcal-root button { cursor: pointer; }");