The problem here might be the image file type.
Example: ext-theme-classic
form images use .gif
while ext-theme-neptune
uses .png
. So if you want to use neptune's form images, copy the images to your custom theme then find an image editor and change its file type from png to gif.
Or you need to override Component.scss
(sass/var/) then set the following:
from:
$image-extension: 'gif' !default;
to:
$image-extension: 'png' !default;
Note: I haven't actually tried the second option but I think it should work :)