When you connect your computer to the LED, there should be no black borders (apart from the physical black edge on the screen). What we mean by this is that your image should fill the whole of the screen, just as the onboard Android does. If it does not, then you need to change the display settings. In the video below you will see that the image does not fit on the screen and we use the display settings menu to change this.
With your computer displaying on the screen, swipe up to access the Android Swipe Menu and chose the settings option. Scroll down until you see Display Settings. Then cycle through the options until you find the setting that fits your computer the best. You should find that 16:9 or P2P is the best option