Please have a look at Customize Styling Using Calypso - Studio Pro 8 How-to's | Mendix Documentation
You can give classes in mendix studio pro which you can use in your scss. These scss files will be compiled to css using calypso.
Hope this helps!
Only use !important until you have explored all other options to find the right selector. To find the right selector, in your browser (Firefox in my case) right click you tab and open the inspector (F12), in the inspector-window select the tab, again rightclick and select Copy → Copy CSS selector. This will likely get you:
.nav-tabs > li:nth-child(3)
If that is not specific enough, do it again, but now Copy → Copy CSS Path. This gets you the entire path. Take css names form the end of the path (probably ul.nav.nav-tabs.mx-tabcontainer-tabs li) and keep adding them to your selector until your selector is specific enough.
Hope it helps.