This PR updates the style of Aztec gallery dialog.

To test:

• Using Aztec editor, open Add Media dialog and navigate to WordPress media library.
• Select multiple images and confirm your selection.
• Check that the resulting dialog looks good in both dark and light modes.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.