It is A BetterDiscord Plugin to change the Background with help from this Theme NotAnotherAnimeTheme
You can remove this part as this sets the plugin:
/*
Theme Background
*/
--theme-background-image: url(https://i.imgur.com/j1Z6HVc.jpg);
Install:
Ensure BetterDiscord is installed.
Put the js and json file in the plugins folder of Betterdiscord
Install the Theme NotAnotherAnimeTheme
Put the plugin https://raw.githubusercontent.com/rauenzi/BDPluginLibrary/master/release/0PluginLibrary.plugin.js in the plugins folder.
Restart the Discord client either by right clicking the tray icon and clicking "Quit Discord" or by pressing Ctrl+R inside the client.
Go to User Settings in Discord / Plugins / Select Background v0.2.9 by Letsplaybar
To set how long an image should stay or remove / add images go to options / plugins / Background / settings. Here you have 4 forms to change the necessary settings.
Time in Seconds.