themes/bthree/src/styles/_wp-block.sass

@@ -315,6 +315,8 @@
     margin: 0
 
 .block-words-cloud-categories
+  @extend .theme-dark
+
   --words-cloud-column-count: 1
   column-count: var(--words-cloud-column-count)
   column-gap: 1
@@ -333,6 +335,9 @@
     color: var(--color-text-primary)
 
   > div
+    // Re-assign variable --box-bg-color to prevail in local scope
+    --box-bg-color: var(--color-bg-primary)
+
     +box-card
     +margin(3, bottom)