Make Element welcome page text readable on the dark theme

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>

roles/custom/matrix-client-element/templates/welcome.html.j2

@@ -167,6 +167,15 @@ we don't have an account and should hide them. No account == no guest account ei
     }
 }
 
+/* Element Web (with its default use_system_theme behavior) renders its dark background
+   when the system prefers a dark color scheme, so the welcome text follows the same signal.
+   #ebeef2 is the light text color Element uses with the dark theme. */
+@media (prefers-color-scheme: dark) {
+    .mx_Parent {
+        color: #ebeef2;
+    }
+}
+
 </style>
 
 <div class="mx_Parent">