chore: WIP scope theming css variables with packages

[nnaydenow]

Scope and package --sap CSS variables with the web components package. Use the default value initially for each theme, and apply the --sap CSS variables only if an external theme is detected.

Page with Web Components

• Uses scoped --sap CSS variables.
• Each variable is derived from the original CSS variable name, with a default value per theme provided by @sap-theming/theming-base-content as a fallback.

Page with Web Components + Additional @sap-theming/theming-base-content

• Also uses scoped --sap CSS variables.
• In this setup, the active theme is determined by the loaded theming base content, regardless of the theme set for the web components.
• The theme set for the web components is only used as a fallback when a CSS variable is missing in the loaded theming base content.

[ui5-webcomponents-bot]

🚀 Deployed on https://pr-12491--ui5-webcomponents-preview.netlify.app