[12.x] Modernize default mail template

[tygoegmond]

This PR refreshes Laravel's default mail template with a cleaner, more modern look that aligns with the starter kits.

Changes

• Updated color palette to neutral grayscale tones
• Increased border-radius on cards (2px → 12px) and buttons (4px → 8px)
• Improved typography with semibold headings and better line-height
• Replaced panel left-border accent with subtle rounded box
• Refined shadows and spacing throughout
• Adjusted button padding for better proportions

Before / After

Notes

• Fully backwards compatible, no markup changes required
• Maintains email client compatibility (Outlook, Gmail, Apple Mail)
• Only CSS changes in default.css

[shaedrich]

I don't know about this. The changes are minimal and what you changed didn't look that bad nor incredibly outdated before.

[tygoegmond]

I don't know about this. The changes are minimal and what you changed didn't look that bad nor incredibly outdated before.

The main goal is to align it with the design used in the starter kits. Same neutral colors, rounded corners, etc. Makes it feel more cohesive out of the box.

[shaedrich]

Ah, okay. That context would have been helpful. Could you change the title to this or add it to the description?

Would there be a good way to share this CSS from the starter kits directly?

[taylorotwell]

A good idea - I took my own stab at it.

#57987