There are a couple of approaches you could try, but the easiest is probably the Style macro.
Note: It requires a knowledge of CSS and that you are comfortable applying a manual override to Atlassian's styling.
This won't add a new style to the text editor, but would allow you to apply what ever styling you like to some text on the page.
If you want to change the styling of the page tile across an entire space, once you've worked out the right style to tweak, add that macro to Space Tools > Look and Feel > Sidebar, header and footer.
If you want to make this apply to your entire site, you'll need to add it to each space.
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