New Horizontal Category Tabs

Looking at the new CSS rules Ecwid use for the horizontal tabs, I see that these two rules are probably what you need to override:

this rule sets text color:

.horizontal-menu-container.horizontal-desktop .horizontal-menu-item > a{
  color:#4473A4;
}

this rule sets bg color and text color for hover:

.horizontal-menu-container.horizontal-desktop .horizontal-menu-item.horizontal-menu-item--hover > a {
  background: #75ABAE;
  color: #fff
}