feat: Use the colors of the administration theme, in light and dark mode

Problem/Motivation

The welcome card uses a fixed blue gradient and the cards use fixed colors: they ignore the accent color chosen in the administration theme and its dark mode.

Proposed resolution

• Build the welcome card, the stat cards and the status badges on the colors of the administration theme: the core Default Admin theme first, then Gin and Claro.
• Use the selected accent color, and follow light and dark mode where the theme has it.

Remaining tasks

• ✅ Implement
• ❌ Review

AI-Generated: Yes