fix(ui): use correct dark theme selector in Dialog (#33898)

* fix(ui): use correct dark theme selector in Dialog

The Dialog component used [data-theme='dark'] while every other BUI
component and the global token system uses [data-theme-mode='dark'].
This mismatch meant the dark mode background rule never matched.

Signed-off-by: David Josefson <david.josefson@neo4j.com>

* Update .changeset/fix-dialog-dark-theme-selector.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: David Josefson <david.josefson@neo4j.com>

---------

Signed-off-by: David Josefson <david.josefson@neo4j.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

.changeset/fix-dialog-dark-theme-selector.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed dark mode background for Dialog component by correcting the theme attribute selector from `data-theme` to `data-theme-mode`.
+
+**Affected components:** Dialog

packages/ui/src/components/Dialog/Dialog.module.css

@@ -30,7 +30,7 @@
     z-index: 1000;
   }
 
-  [data-theme='dark'] .bui-Dialog {
+  [data-theme-mode='dark'] .bui-Dialog {
     background: rgba(0, 0, 0, 0.5);
   }