[base] Update default CSS variables

[mariuslundgard]

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe: Improvement / design update.

Does this change require a documentation update? (Check one)

• Yes
• No

Current behavior

The default Studio theme does not use the Sanity color scheme.

Description

The CSS variables now use the Sanity color schema, by overriding specific values in the override.css file.

Test script

• Try to find bugs related to colors and spacing
• See how everything works with custom themes by adding an override to packages/test-studio/sanity.json:

    {
      "implements": "part:@sanity/base/theme/variables/override-style",
      "path": "src/themes/dark.css"
    }

// or

    {
      "implements": "part:@sanity/base/theme/variables/override-style",
      "path": "src/themes/hotdog.css"
    }

Note for release

Checklist

• I have read the Contributing Guidelines
• The PR title includes a link to the relevant issue
• The PR title is appropriately formatted: [some-package] PR title (#123)
• The code is linted
• The test suite is passing
• Corresponding changes to the documentation have been made

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[bjoerge]

LGTM!