fix: node preview background color (#6768)

## Summary

Changes node previews to use background color from color palette instead
of the menu color.

| Before | After |
| ------ | ----- |
| <img width="1700" height="1248" alt="Selection_2345"
src="https://github.com/user-attachments/assets/cc1d0e97-9551-4f88-8e92-4fe6bcdd8c21"
/> | <img width="2144" height="1138" alt="Selection_2347"
src="https://github.com/user-attachments/assets/16e64be3-3623-4900-ad18-c599a1aee59b"
/> |

Fixes https://github.com/Comfy-Org/ComfyUI_frontend/issues/6576

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6768-fix-node-preview-background-color-2b16d73d365081868644e1e6b7c7e3be)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Alexander Brown <drjkl@comfy.org>
Co-authored-by: GitHub Action <action@github.com>

src/components/node/NodePreview.vue

@@ -3,7 +3,7 @@ https://github.com/Nuked88/ComfyUI-N-Sidebar/blob/7ae7da4a9761009fb6629bc04c6830
 -->
 <template>
   <LGraphNodePreview v-if="shouldRenderVueNodes" :node-def="nodeDef" />
-  <div v-else class="_sb_node_preview">
+  <div v-else class="_sb_node_preview bg-component-node-background">
     <div class="_sb_table">
       <div
         class="node_header mr-4 text-ellipsis"
@@ -200,7 +200,6 @@ const truncateDefaultValue = (value: any, charLimit: number = 32): string => {
 }
 
 ._sb_node_preview {
-  background-color: var(--comfy-menu-bg);
   font-family: 'Open Sans', sans-serif;
   color: var(--descrip-text);
   border: 1px solid var(--descrip-text);