You might need to contact Mino_dev on Discord about that.

I do have a trick you could try to kinda re-add the lines if you happen to be introverted and don't feel like talking: Rip the lineart from the sprites directly, increase their contrast/brightness/hue, alongside some minor erasing/redrawing so it's fully black and white.

Paste the final image onto the Defender's .clip file as a separate layer, with a layer effect of "multiply" and resize and adjust it to fit onto the Defender to get the lineart. You can probably use a sharpen of blur effect so it doesn't look quite off, but you'll resize the image to the small 500(normal)/250(low quality) sprite height that it likely won't make as huge of a difference as you'd think it would in the end result.

You could also delete all the white from the picture before pasting it, but I find that it tends to have little bits of "bleeding" with stray non-white but also non-black pixels and multiply tends to be more consistent.

Otherwise, best to contact the developer regarding this.