refactor: change theme variable name from green to brand - #154

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57, actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/