Update Modal design to reflect latest SLDS blueprint

[xulingzhihou]

SLDS blueprint has updated modals to have a new white background behind the 'X' to address accessibility concerns for color contrast. Currently in design system react the button is coded to be a reverse button which results in the 'X' to be invisible.

New design:

Issue:

• If you are working on the problem. Issues may be closed within 30 days if no one is assigned. Maintainers work on this project 20-30 hours per week and have their own prioritized backlog.
• How to duplicate a bug. Line numbers or code to paste into the documentation site examples is helpful.
• If you are a Salesforce employee. You should also create a duplicate work item in your internal ticket system and link to this issue.
• If you plan to work on a new component. Please propose props after reviewing at newly written components and docs/codebase-overview.md.

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This project uses SLDS markup, but you should submit HTML/CSS issues to https://github.com/salesforce-ux/design-system.