Change Page Title since WP Update to 5.0

For one of my pages I use the “Flashlight” theme. Since the update to WP 5.0, which also includes the Gutenberg editor, I can no longer access the shortcodes. I have now installed the Classic Editor plugin, this is how it works again.
But previously, the page title was always automatically used as a page header in large font size combined with a horizontal line separating it from the text. The headline was set in the middle. That looked really convincing. Since the WP update, the title is only displayed in normal text size in the frontend. I just saw it in your demo site too – some sites are in the old well looking header style and some are without any format. Why is that, and how can I change that?