Below Css has been used to design the tabpane
#testDatesTab .tab-selected-line {
-fx-background-color: #0059A9;
}
#testDatesTab .headers-region {
-fx-background-color: #F5F6F8;
}
#testDatesTab .tab-header-background {
-fx-background-color: white;
}
#testDatesTab .control-buttons-tab{
-fx-background-color:white;
}
#testDatesTab .control-buttons-tab .tab-down-button{
-fx-background-color: #0059A9 ;
}
#testDatesTab *.tab-header-area {
-fx-padding: 0 0 0 0;
}
Using those style lines you should not have problems, maybe if you have something else in your css it is what is causing that problem, look I just made an example. Add your full css.