fix(button): idle icon color not updated

[simeonoff]

Related to https://github.com/IgniteUI/igniteui-angular-samples/issues/3476

The icon color should change the way the text changes if the $background theme property is updated.

Additional information (check all that apply):

• [x] Bug fix
• [ ] New functionality
• [ ] Documentation
• [ ] Demos
• [ ] CI/CD

Checklist:

• [x] 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