refactor: use Codex design tokens for template default colours
Replaced hardcoded hex colour values with Codex semantic design tokens across all banner templates.
Existing saved banners keep their stored values and are unaffected.
Bug: T429069
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Replaced hardcoded hex colour values with Codex semantic design tokens across all banner templates.
Existing saved banners keep their stored values and are unaffected.
Bug: T429069