How to style a custom <vaadin-select>

I’m not sure I understand, but in a Fusion (18.0.6) project, when I add a Select (like the first example [here]
(https://vaadin.com/docs-beta/latest/ds/components/select/)), give it a theme attribute: theme="my-styled-select", and I add the following style to shared-styles.js

[theme='my-styled-select']
 vaadin-item[selected]
 {
        background-color: yellow;
      }

The selected item has a yellow background alright