Cleaned a bit more

tools/editor/styles/components/tool-preview.css

@@ -1,8 +1,15 @@
 /* Component – Tool preview */
 
 .tool-preview {
+  background: var(--bg-surface);
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
   flex: 1;
   position: relative;
   min-height: 300px;
   transition: border-color var(--speed);
 }
+
+.tool-preview.drag-over {
+  border-color: var(--accent-dim);
+}