You can add 3 form of icons to your support center labels: Main, hover and active.
By default, If you have only a main icon the icons for the hover and active states will be taken from it.
To add an icon go to: Knowledge => All Articles => Click on the drop down label icon => Manage labels.
At the Content labels window, choose the label you want or add a new one. => Click on the icon and upload the icon image from your computer.
To upload a specific icon for active or hover, uncheck the checkbox at the icon field and add the icon.
*Please note that