Question

So is there any way to remove the gray area behind the tab(s):

I’ve tried to do this with CSS, but didn’t find how.

Answer 1

JavaFX has a built-in default CSS sheet named caspian.css. Please read this post: https://stackoverflow.com/a/10149050/682495.
To change the default CSS, add this selector to your CSS file and customize it:

.tab-pane *.tab-header-background {
    -fx-background-color: -fx-outer-border, -fx-inner-border, derive(-fx-color, -20%);
    -fx-effect: innershadow(two-pass-box , rgba(0,0,0,0.6) , 4, 0.0 , 0 , 0);
}