This tool comes with pre-added battery icons (Red, Orange, Blue ... then you can also add any ICO files for battery levels less than 25%, above 50%, above 75%, etc. Use this link and grab the ...

You can set a dark or any icon of your choice – but the images have to me in the .ico format. Read: How to change File and Folder Icons. As an example, let us change the icon to a dark one.