Below is a screenshot (taken in IE10) which contains some different Tabstrip CSS customizations/designs for the Application Theme Platinum.
Feel free to use any of them.
Attached you can find the css definitions for the custom css classes.
I have created the following 8 custom css classes:
To use a design: copy the corresponding code into your custom css file which must be of course also linked to your design studio bi app and add/assign the css class name to your tabstrip component (clear browser cache if needed and view result in browser).
Related blog post: How To: Create a manila folder like vertical tabs using CSS