I am new to SharePoint online and I was wondering if you can help me with my question:
- How I can change the top navigation style, for-instance font-size, color, font-family?
On communication sites, you can change the theme and header, layout of your navigation and add a footer to your site.
You can change the navigation styling in SharePoint online modern experience from Settings >> Change the look.
Reference: Change the look of your SharePoint site
If you are not happy with the standard themes, you can also define custom themes, custom styles and colors to sites.