Bump cssnano from 7.1.9 to 9.1.0 in /theme

Bumps [cssnano](https://github.com/cssnano/cssnano) from 7.1.9 to 9.1.0.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@7.1.9...cssnano@9.1.0)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-version: 7.1.7
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

theme/package.json

@@ -39,7 +39,7 @@
     "autoprefixer": "^10.4",
     "babel-loader": "^9.0",
     "core-js": "^3.32",
-    "cssnano": "^7.1.1",
+    "cssnano": "^9.1.0",
     "eslint": "^10.0.3",
     "globals": "^17.4.0",
     "postcss": "^8.5.10",