First you need to understand how to customize Mendix with styling:
https://docs.mendix.com/howto/front-end/customize-styling-new
I would add a class so I have control over which tabs the new styling will be applied.
You can add the following selector:
.custom-tab-styling.mx-tabcontainer .mx-tabcontainer-tabs>li>a {
color: red;
padding: 2em;
}
The styling will be applied to the tab containers where you have added the class ‘custom-tab-styling’