The default colors for your tab buttons pull from your section theme’s color variables. To change them, use the below CSS. Delete any lines that you wish to remain as the default.
/**
* Tabs Plugin
* From Will-Myers.com
* Tab Button Colors
**/
div.wm-tabs {
// Default Buttons
--tab-button-background: transparent;
--tab-button-color: black;
// Active Tab Buttons
--tab-button-active-background: transparent;
--tab-button-active-color: white;
// Indicator Color
--active-indicator-color: black;
}