To change the tab borders, use the below CSS. Delete any lines that you wish to remain as the default. 

/**
 * Tabs Plugin
 * From Will-Myers.com
 * Tab Button Borders
**/
div.wm-tabs {
  // Default Buttons
  --tab-button-border-radius: 5px;
  --tab-button-border-width: 0px;
  --tab-button-border-color: currentColor;

  // Active Tab Buttons
  --tab-button-active-border-color: currentColor;

  // Indicator Borders
  --active-indicator-border-radius: 5px;
  --active-indicator-border-color: currentColor;
  --active-indicator-border-width: 0px;
}
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