It’s being set in oceanic/templates/css/oceanic-header-standard.css.
Do you know how to use your browser’s developer tools? By using the developer tools you should be able to click any element on your site and see both what that element’s style is set to as well as what file is setting it. Here’s an article on inspecting elements with Chrome’s Developer tools, which are great:
https://developer.chrome.com/devtools/docs/dom-and-styles