Hello Mary,
that rule you posted would affect all the texts in the quiz player, and that's not what we want. Also, since CSS rules have priorities (more specific rules have more priority), it probably won't be applied to most texts.
You should use more specific rules, something like:
ion-content.site_mod_quiz-player h3 {
font-size: 20px;
}
To apply it, it's as you said. You upload it to your Moodle site and then paste the link in Mobile appearance > mobilecssurl.