Add Dashicons to your Customizer Panels. You can find the dashicon CSS classes here: https://developer.wordpress.org/resource/dashicons/#id

add-dashicons-to-wordpress-customizer.php

<?php
/**
 * Enqueue the stylesheet.
 * http://aristeides.com/blog/modifying-wordpress-customizer/
 */
function my_enqueue_customizer_stylesheet() {

    wp_register_style( 'my-customizer-css', YOUR_PLUGIN_URL. 'assets/css/customizer.css', NULL, NULL, 'all' );
    wp_enqueue_style( 'my-customizer-css' );

}
add_action( 'customize_controls_print_styles', 'my_enqueue_customizer_stylesheet' );

compiled-css.css

/*!
 * The panel names are based on the IDs you assign to them. Use your Firebug/Chrome inspector to quickly find them.
 */

#customize-controls .accordion-section-title:before,
#customize-controls .panel-title:before {
  -moz-osx-font-smoothing: grayscale;
  display: inline-block;
  font-family: dashicons;
  font-size: 20px;
  font-style: normal;
  font-weight: 400;
  height: 20px;
  line-height: 1;
  text-align: center;
  text-decoration: inherit;
  vertical-align: top;
  width: 20px;
  opacity: 0.6;
}
#customize-controls .accordion-section-title:before:hover,
#customize-controls .panel-title:before:hover {
  opacity: 1.0;
}
#customize-controls .panel-title:before {
  left: 2px;
  margin-right: 5px;
  position: relative;
  top: 4px;
}
#customize-controls #accordion-panel-wf_plus_stylekit_panel .panel-title:before,
#customize-controls #accordion-panel-wf_plus_stylekit_panel > h3:before {
  content: "\f100";
}
#customize-controls #accordion-panel-wf_plus_layout_panel .panel-title:before,
#customize-controls #accordion-panel-wf_plus_layout_panel > h3:before {
  content: "\f135";
}
#customize-controls #accordion-panel-wf_plus_typography_panel .panel-title:before,
#customize-controls #accordion-panel-wf_plus_typography_panel > h3:before {
  content: "\f205";
}
#customize-controls #accordion-panel-wf_plus_buddypress_panel .panel-title:before,
#customize-controls #accordion-panel-wf_plus_buddypress_panel > h3:before {
  content: "\f336";
}

dashicons-less.less

  .accordion-section-title:before,
  .panel-title:before {
    -moz-osx-font-smoothing: grayscale;
    display: inline-block;
    font-family: dashicons;
    font-size: 20px;
    font-style: normal;
    font-weight: 400;
    height: 20px;
    line-height: 1;
    text-align: center;
    text-decoration: inherit;
    vertical-align: top;
    width: 20px;
    opacity: 0.6;
    
    &:hover {
      opacity: 1.0;
    }
  }
  

  .panel-title:before {
    left: 2px;
    margin-right: 5px;
    position: relative;
    top: 4px;
  }
  

  #accordion-panel-wf_plus_stylekit_panel {
    .panel-title:before,
    > h3:before {
      content: "\f100";
    }
  }
  

  #accordion-panel-wf_plus_layout_panel {
    .panel-title:before,
    > h3:before {
      content: "\f135";
    }
  }
  

  #accordion-panel-wf_plus_typography_panel {
    .panel-title:before,
    > h3:before {
      content: "\f205";
    }
  }
  

  #accordion-panel-wf_plus_buddypress_panel {
    .panel-title:before,
    > h3:before {
      content: "\f336";
    }
  }