Kathryn on "Flato/Plane Theme"

Which "header text" do you mean exactly? If you're referring to your post and page titles, something like this should do the trick:

.entry-header .entry-title {
  font-size: 2.5rem;
}

Don't edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.

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.

If you need further help, please start a new thread and add this in the tags read of the post:

wpcom-theme