gitlab-org--gitlab-foss/app/assets/stylesheets/components/rich_content_editor.scss

// Overrides styles from ToastUI editor
.tui-editor-defaultUI-toolbar .toolbar-button {
  color: $gl-gray-600;
  border: 0;

  &:hover,
  &:active {
    color: $blue-500;
    border: 0;
  }
}