fix(ui): improve dialog backdrop appearance in dark mode

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/honest-lamps-occur.md

@@ -0,0 +1,5 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed dialog backdrop appearance in dark mode.

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

@@ -23,7 +23,7 @@
     left: 0;
     width: 100%;
     height: 100%;
-    background: rgba(232, 232, 232, 0.8);
+    background: color-mix(in srgb, var(--bui-gray-2) 80%, transparent);
     display: flex;
     align-items: center;
     justify-content: center;