fix(ia): dashboard header icon background

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

The variable being used to render the Newspack icon for the dashboard wizard (--newspack-ui-color-primary) is not available, causing it to appear as a blank square:

Removing it allows for the Newspack icon to render the same way for all wizards, which is the new darker blue.

How to test the changes in this Pull Request:

1. While on epic/ia, confirm that the wizard on "Newspack -> Dashboard" doesn't render the Newspack icon correctly
2. Checkout this branch, refresh the page and confirm the header renders as expected
3. Navigate to "Audience -> Setup" and confirm it renders the same

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?