fix(core/modal): fix paddings of custom variant

[matthiashader]

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)
• Build (yarn build) was run locally and any changes were pushed
• Unit tests (yarn test) were run locally and passed
• Visual Regression Tests (yarn visual-regression) were run locally and passed
• Linting (npm lint) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bug fix
• Feature
• Refactoring (no functional changes, no API changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Currently the padding of the modal, modal-header, modal-content and modal-footer differ from the figma definition.

GitHub Issue Number: N/A

What is the new behavior?

The paddings have been adapted in order to match the figma definition.

Does this introduce a breaking change?

• Yes
• No

Testing

Other information