Hey Steffen,
Which theme are you using? If you’re using Valo, this should help you define the font-size:
https://vaadin.com/book/-/page/themes.valo.html
.
Otherwise, it sounds like a specificity issue. Have you read this?
https://vaadin.com/book/-/page/themes.css.html
. In other words, I think you might just need to be more specific with your selectors.