@BadSeed You can use these .css modifications:

/* Aktiver Tab Color - made by Luetage on the Vivaldi Forums  */

#browser .tab-position .tab.active.active {
  color: #910303 !important;
}

/* change the font color of the active tab - made by nomadic on the Vivaldi Forums */

#browser .tab-position .tab.active .title {
  font-family: "Comic Sans MS", sans-serif; /* change the font used */
  font-size: 12px; /* increase the size */
  font-weight: 500;
  color: #910303; /* text color */
}