Your site looks responsive, but the word “Hengelsportvereniging” is a bit too long to fit on a phone’s screen size. Is that what you’re referring to, or is there something else that doesn’t look right?
You could make the site title smaller on small screens if you wanted to get around the issue. Something like:
@media screen and (max-width: 440px) {
.site-title {
font-size: 26px;
}
}
Don’t edit the theme files directly, otherwise your changes will be overwritten every time the theme is updated to the latest version.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.