fix(cssHelper): narrow background→backgroundColor to ConditionalExpression only

The previous check used `condition === "always"` which also matches
destructured defaults (e.g. `({ bg = "url(...)" }) => background: ${bg}`).
Adding a `callArg.type === "ConditionalExpression"` guard ensures only
theme-resolved conditionals (where both branches are color tokens) get
the backgroundColor conversion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: skovhus

src/internal/lower-rules/template-literals.ts

@@ -491,10 +491,17 @@ export function resolveTemplateLiteralBranch(
         });
         continue;
       }
-      // Handle `background: ${expr}` shorthand → backgroundColor dynamic entry
-      // Only apply when the expression is a theme-based resolution (condition: "always"),
-      // not for simple prop passthroughs (e.g. background: ${props.background}).
-      if (propName === "background" && !prefix && !suffix && resolved.condition === "always") {
+      // Handle `background: ${expr}` shorthand → backgroundColor dynamic entry.
+      // Only apply for theme-resolved conditionals (ConditionalExpression with condition: "always"),
+      // where both branches are theme color tokens. Destructured defaults also set condition: "always"
+      // but produce Identifier/other node types and may carry non-color values (gradients, images).
+      if (
+        propName === "background" &&
+        !prefix &&
+        !suffix &&
+        resolved.condition === "always" &&
+        resolved.callArg.type === "ConditionalExpression"
+      ) {
         dynamicEntries.push({
           jsxProp: resolved.jsxProp,
           stylexProp: "backgroundColor",