--- a/toolkit/components/extensions/parent/ext-theme.js
+++ b/toolkit/components/extensions/parent/ext-theme.js
@@ -80,17 +80,20 @@ class Theme {
       this.loadIcons(details.icons);
     }
 
     if (details.properties) {
       this.loadProperties(details.properties);
     }
 
     // Lightweight themes require accentcolor and textcolor to be defined.
-    if (this.lwtStyles.accentcolor &&
+    // If lightweight themes have additionalBackgrounds, it should have no more than 15 images
+    if (this.lwtStyles.additionalBackgrounds && this.lwtStyles.additionalBackgrounds.length > 15) {
+      this.logger.warn("Your theme includes more than 15 images in images: 'additional_backgrounds'");
+    } else if (this.lwtStyles.accentcolor &&
         this.lwtStyles.textcolor) {
       if (this.windowId) {
         windowOverrides.set(this.windowId, this);
       } else {
         windowOverrides.clear();
         defaultTheme = this;
       }
       onUpdatedEmitter.emit("theme-updated", this.details, this.windowId);