Files app: Fix styles for disabled button

The disabled state wasn't applied to disabled button because the
selector was less specific. Fixed by adding a rule with "files-ng" to
increase the specificity and using the semantic color.

FIXED: 1116401
Change-Id: Ib3bb638d82b2bf488eca34894c86c63550ccb63c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359590
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798511}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -1403,6 +1403,15 @@ body.files-ng .dialog-footer button {
   color: rgb(168, 168, 168);
 }
 
+body.files-ng .dialog-footer cr-button[disabled],
+body.files-ng .dialog-footer cr-button[disabled]:hover,
+body.files-ng .dialog-footer button[disabled],
+body.files-ng .dialog-footer button[disabled]:hover {
+  background-color:
+      var(--cros-button-background-color-primary-disabled);
+  color: var(--cros-button-label-color-primary-disabled);
+}
+
 body:not(.files-ng) .dialog-footer .primary,
 body:not(.files-ng) .dialog-footer .primary:hover {
   background-color: rgb(51, 103, 214);