Fix 'Open' button text color for non files-ng

Selector specificity due to selector lengths for files-ng caused the
selector for check-select 'Open' button color to be overridden.

Change the non files-ng selector 'Open' button color case to not be
applied when check-select mode is active.

Bug: 1068556
Change-Id: I5fd36169941bcb4a1fad111d090b6ef728761aff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147138
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759055}

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

@@ -610,8 +610,8 @@ html.focus-outline-visible .dialog-header.files-ng button:not(:active):focus {
   display: none;
 }
 
-.dialog-header:not(.files-ng) button,
-.dialog-header:not(.files-ng) button:hover {
+body:not(.check-select) .dialog-header:not(.files-ng) button,
+body:not(.check-select) .dialog-header:not(.files-ng) button:hover {
   color: white;
 }