Atlas provides several classes that change the display format of tabs. One that I use often is tab-pills, which will render your tabs like this:
You can see all of the classes here: https://atlas.mendix.com/p/widget_tabcontainer/5910974510924179
Hope that helps,
Is it possibble to handle this in JS? If true, then you could take a look to the HTML/JS Module on the App Store.