The level in a tab is lost when is used the bootstrapbase theme.

Normally, the renderer set the level in a CSS class with the next code:
$str .= html_writer::start_tag('ul', array('class' => 'tabrow'. $tabobject->level));

But the renderer en bootstrapbase overwrite this. Normally, we can use the class "tabrow0" or "tabrow1" in order to set differents styles in differents levels of rows in a tabs menu.