When this property is set to a value of 'false', it means the default style for the thumbnails and its contents is not required. Under this condition, your web page containing the ImGearThumbnailList control must include a CSS file that contains definitions for the following named CSS classes:

• .imGearTNItem, .imGearTNSelected

  In this example, the CSS class group defines the common property styles for the thumbnails that are in "unselected" and "selected" states. Alternatively, you can define two separate CSS classes, .imGearTNItem and .imGearTNSelected. Each containing all the properties that you want them to contain. The end result is the same.

  Example:
  .imGearTNItem, .imGearTNSelected
  {
  width:100px;
  height:160px;
  background-color:gray;
  float:left;
  overflow: hidden;
  border: 1px solid black;
  margin: 2px;
  }
• .ImGearTNSelected

  This CSS class defines the background for the thumbnail items in the selected state.

  Example:
  .imGearTNSelected strong>
  {
  background-color:#ffBB00;
  }
• .imGearTNItemCaption

  In this example, the CSS class defines details for thumbnail items caption.

  Example:
  .imGearTNItemCaption
  {
  left:0px;
  font-family: arial,sans-serif;font-size:8pt;text-align: center;
  font-size: 16px;
  top:20px;
  width:100px;
  height:20px;
  }
• .imGearTNImage

  This CSS class defines thumbnail image style.

  Example:
  .imGearTNImage
  {
  width:120px;
  height:100px;
  position:relative;
  overflow: hidden;
  margin: 2px;
  }
• .imGearTNItem:Hover

  This CSS class defines thumbnail item style when mouse hovers over it.

  Example:
  .imGearTNItem:Hover
  {
  border:2px solid #ffBB00;
  }

Reference