better prompt strings for flutter theme agent

[Rahul-Lashkari]

Description of the change

better prompt definitions for the Flutter Theme Agent

Motivation

better adherence from the model

Type of change

Checklist

• I have performed a self-review of my code.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.