Update code examples and tutorial for theming-v2

[szuperaz]

Motivation When themeing-v2 is implemented we should make sure our documentation is aligned with the changes

Proposed solution

• Update existing code examples in the docs that use theme variables (if any)
• Update tutorial: to use theme-v2
• Update screenshots in docs (if any)
• Update theming guide with examples and screenshots -> https://github.com/GetStream/stream-chat-css/blob/theming2/docs/theming/introduction.mdx

Acceptance Criteria A set of predefined requirements that must be met to mark a user story complete.