feat(scrollbar): add scrollbar theme

[simeonoff]

Closes #6675

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code
• This PR includes API docs for newly added methods/properties
• 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
• This PR includes behavioral changes and the feature specification has been updated with them

[desig9stein]

Changing the size of the scrollbar has unexpected behavior in the igx-grid component. Increase the size from 16px to 20px and observe.

[simeonoff]

@simeonoff If I add a theme using one of the predefined mixins, for example, igx-dark-theme everything is working correctly but if I try to use the igx-theme mixin and pass a dark palette and dark schema the scrollbar remains white.

I can't reproduce.