Modify context menu styles

[jnyfil]

I am uncomfortable using the default context menu of litegraph, so I made a small modification to improve it.

Before:

After:

Now, The button area is enlarged, which should help prevent some of the mis-selection with the auto-open.

https://github.com/comfyanonymous/ComfyUI/assets/75435724/16328804-10d5-4210-a8e5-eb86284c3d00