Add support for dark and light theme

[fantonangeli]

Description

Implement the support to the light and dark theme in the editor according to the user system settings

Motivation

Follow the user preferences to show the right editor's theme

Proposed Implementation

The goal:

• follow the browser settings by default
• implement an attribute in the component to override this behaviour and force a specific theme

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.

[fantonangeli]

Reactflow colorMode has been introduced in v12: https://github.com/xyflow/xyflow/releases/tag/%40xyflow%2Freact%4012.0.0
We need to have a Reactflow component with the latest version before starting this task.
cc @lornakelly @handreyrc