fix(theming): restore gruvbox as default via Default() helper

internal/theming/theming_test.go

@@ -192,3 +192,9 @@ func TestApplyEachThemeWritesStarship(t *testing.T) {
 		})
 	}
 }
+
+func TestDefaultIsGruvbox(t *testing.T) {
+	if got := Default(); got.ID != "gruvbox" {
+		t.Fatalf("Default().ID = %q, want gruvbox", got.ID)
+	}
+}