#7661404 Pattern Assembler – Update copy of preview placeholder

Proposed Changes

• Update design for preview placeholder
• Add paddings for mobile preview

Testing Instructions

• Go to/setup/site-setup?siteSlug=<your_site>
• Continue until you land on the Design Picker
• Click the “start designing” button at the end of the Design Picker
• Verify whether the new styles of the preview placeholder look good for all viewports

Before	After

On the mobile viewport

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

Related to #70692