fix: check if theme/styles.css exists before including it

[jbroma]

Summary

This PR makes styles.css optional for the preset to work

Test plan

• - tester works

---

Note

Conditionally resolve globalStyles via a new helper so theme/styles.css is optional.

• Preset config (packages/preset/src/preset.ts):
  • Add getGlobalStyles(context) to return theme/styles.css path only if it exists.
  • Update globalStyles to use getGlobalStyles(context), making global styles optional.

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