dark-mode: set boundlist background

this is only visible when no boundlist items are present, which only
occurs when loading elements for the boundlist (e.g., when adding a
nfs storage)

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>

src/proxmox-dark/scss/extjs/form/_combobox.scss

@@ -4,6 +4,7 @@
 }
 
 .x-boundlist {
+  background-color: $form-field-body-color;
   border-color: $form-field-body-color;
 }