style: extract button styles to mixin

[DavideMininni-Fincons]

Preflight Checklist

• [ ] I have read the Contributing Guidelines for this project.
• [ ] I agree to follow the Code of Conduct that this project adheres to.
• [ ] I have searched the pull request tracker for a Pull Request (PR) that matches the one I want to submit, without success.

Issue

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Pull request checklist

Please check if your PR fulfills the following requirements:

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)

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Changes

Changes in this pull request:

Browsers

I tested the build on the following browsers:

• [ ] Firefox Desktop
• [ ] Chrome Desktop
• [ ] Edge Desktop
• [ ] Safari Desktop
• [ ] Chrome Mobile
• [ ] Safari Mobile

Screen readers

I tested the build on the following browsers:

• [ ] JAWS Firefox Desktop
• [ ] JAWS Chrome Desktop
• [ ] NVDA Firefox Desktop
• [ ] NVDA Chrome Desktop
• [ ] VoiceOver Safari Desktop
• [ ] VoiceOver Chrome Desktop
• [ ] VoiceOver Safari Mobile
• [ ] Android Accessibility Suite Chrome Mobile

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Does this introduce a breaking change?

• [ ] Yes
• [ ] No

Other information