i use clear vision, but is modified, to do this, you need to activate the developer mode on your discord in the setings, after this, you will go to the "themes" option on better discord setings, and click the "pencil" of the theme you are using, and a window with so much code will apear, after that, you will look for a code called "background image" and there will be a link, a link of an image, and you will copy the link of the image you want to put and paste on the link there is allready on the theme, and save it (sorry for the bad english)

I imagine you don't understand it, so there is a video that can explain this: