bugfix checkout page cart icon color

[Karlasa]

Description

On smaller screens checkout page cart icon didn't had hover color and regular color was not implemented through correct variable. (Should be button color, not primary color)

Manual testing scenarios

1. Go to checkout page
2. scale screen down - to see cart icon
3. move mouse over cart icon

Expected result

cart icon color changes

Actual result

cart icon color stays same

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @Karlasa. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.6 release.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.