changing the size of the font in tabs

Add code to style.css in your child theme.

Tab Headers:

.js_active .tab {
font-size: 11px;
}

Tab content font size is handled within the Enfold Settings, its a global font size setting.
Alternatively you can add this code to your style.css in your child theme:

.active_tab_content .tab_inner_content {
font-size: 11px;
}

to change the background {highlight} colour of the tab:

div .main_color .tabcontainer .active_tab {
background-color: #000;
}