Try to replace that code with this one

.wpb-js-composer .vc_tta.vc_general.vc_tta-style-classic.vc_tta-tabs .vc_tta-tab>a {
   color:rgba(255,255,255,0.8)!important;
}

.wpb-js-composer .vc_tta.vc_general.vc_tta-style-classic.vc_tta-tabs .vc_tta-tab.vc_active>a {
   color:white!important;
}

.wpb-js-composer .vc_tta.vc_general.vc_tta-style-classic.vc_tta-tabs-position-bottom .vc_tta-tabs-container:after, .wpb-js-composer .vc_tta.vc_general.vc_tta-style-classic.vc_tta-tabs-position-top .vc_tta-tabs-container:after {
   background: -webkit-gradient(linear,left top,right top,from(rgba(26,26,26,0)),to(#000000))!important;
   background: linear-gradient(to right,rgba(26,26,26,0) 0,#000000 100%)!important;
}