feat(input-group): improve input-group theme

[didimmova]

Closes #15708

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 (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

[didimmova]

Important Disclaimer

If you want to set a box input theme, scope the theme vars under .igx-input-group--box
The same goes for the search input, scope the theme vars under .igx-input-group--search

Otherwise in some cases, the other type inputs can break.
For example using a dark color for the $box-background property will break the border in the border and line inputs