fix(grid): wrap theme-builder selectors in where pseudo selector

[didimmova]

Description

Motivation / Context

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog
• Skills/Agents

Component(s) / Area(s) Affected:

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Test Configuration:

• Angular version:
• Browser(s):
• OS:

Screenshots / Recordings

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified

[Copilot]

Pull request overview

Updates the Grid theme builder’s root selector to wrap all grid component selectors in :where(...), reducing selector specificity while keeping the same matching behavior. This helps theme-generated styles (CSS variables + token overrides) be easier to override downstream without needing higher-specificity selectors.

Changes:

• Replaced the multi-selector list (igx-grid, igc-grid, ...) with a single :where(igx-grid, igc-grid, ...) selector in the grid theme builder.