How to make headings bigger in the bar in Berlin theme

I’m wondering if there is a way to make the headings bigger along the top bar in the Berlin theme? Unfortunately I don’t have access to the back end files of the website as Omeka is provided through my university, so I’m wondering if there is a way to do this with the CSS plugin or something? I am a newbie to HTML5 and CSS, so any help would be much appreciated.

Here is a link to the website:

Thank you.

When you say “top bar” do you mean the page content, or the navigation menu?

Sorry, the navigation menu