Hi @Marco,
you can customize your app bar, but not adding images or a different font unless you want to change the CSS.
Anyway, to customize your app bar open (from your app) [Settings -- User Interface -- Navigation Menus -- Default]
You'll find an xml file to modify.
You can find the instructions to do this at https://dev.splunk.com/enterprise/docs/developapps/createapps/addnavsplunkapp/ or https://dev.splunk.com/enterprise/tutorials/quickstart/changenavigation/
Ciao.
Giuseppe
@Marco just to add to @gcusello 's point, following is an older answer of mine to create CSS extension based custom navigation. https://community.splunk.com/t5/Dashboards-Visualizations/Can-we-implement-cascading-dropdowns-in-a-...