There is a margin set on the submenu to indent the items.
You can override the margin by doing:

    core-submenu::shadow #submenu {
      margin-left: 20px;
    }

To style the item for the submenu, do something like this:

    core-submenu::shadow > #submenuItem {
      color: blue;
    }
    
To style all the `core-item`s in the light DOM:

    polyfill-next-selector { content: 'core-submenu > #submenu > core-item'; }
    core-submenu > core-item {
      color: red;
    }
    
The above will style `Topic