UI Override

Actually you can already override built-in UI elements You can use Internals.overwriteUIStyle() for that. Just create a new Style object, set the properties you want to override, then assign the Style. To get the path to a built-in element, you can use the uidebugger console command - this enables you to hover an element with your cursor and see the path to the element in the top left corner of the screen. Rightclick on an element to copy the path to clipboard. To disable the UI debugger again, hit the x next to the path (of that doesn't work, press ESC, then hit the x).

Unfortunately you can't get the path to certain elements, but in this case, you could provide the layer name as parameter to the "uidebugger" command - this prints all paths (use page up/down to scroll)

Changing the background image of the inventory screen would look like this:

Style style = new Style();
style.backgroundImage.set(TextureAsset.loadFromFile(...));
style.backgroundImageTintColor.set(1f, 1f, 1f, 1f);
Internals.overwriteUIStyle(player, "inventoryLayer/inventoryContainer", style);

Example screenshot: https://www.rising-world.net/a…682-example-internal-jpg/