The playbook-shipped welcome.html hardcoded #2e2f32 for the headline and
welcome text, which is nearly invisible on the dark background Element
renders when the dark theme is active. Element Web's default
use_system_theme behavior keys the theme off the system color scheme, so
switch the text to Element's dark-theme text color (#ebeef2) under a
prefers-color-scheme: dark media query.

Fixes #4838

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>