if you look at vaadin-combo-box\src\vaadin-combo-box.html you can find the “part” attribute: [part=“text-field”]
. This is intended for styling inner component from the outside.
So try using (e.g. from your shared-styles.html file):
<dom-module id="test-combo-box" theme-for="vaadin-combo-box">
<template>
<style>
[part="text-field"]
{
color: red;
font-size: var(--lumo-font-size-l);
}
</style>
</template>
</dom-module>