Hi Ian,
On the BottomBar block by default, there is an OnReady event that will add the class active to the zero-element-based you pass as an input:

You can change on this block the icons and texts you want for your menu bars:

Being that said, you can check the CSS selector to edit those, like in this example:

The CSS follows below, you can change the color and style it the way you want and add it to your Theme:
.bottom-bar .active .bottom-bar-item-icon,
.bottom-bar .active .bottom-bar-item-text {
color: red;
}
Hope it helps.
Regards,
João