Bump pbkdf2 from 3.1.2 to 3.1.3 in /theme

Bumps [pbkdf2](https://github.com/crypto-browserify/pbkdf2) from 3.1.2 to 3.1.3.
- [Changelog](https://github.com/browserify/pbkdf2/blob/master/CHANGELOG.md)
- [Commits](browserify/pbkdf2@v3.1.2...v3.1.3)

---
updated-dependencies:
- dependency-name: pbkdf2
  dependency-version: 3.1.3
  dependency-type: indirect
...

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Author: dependabot[bot]