Is selling website design templates, which may be used to display images of unveiled women, knowing that other similar templates exist, considered a sin and its earnings forbidden? And is technical assistance permissible if images of unveiled women are discovered within the displayed content?

There is no harm in making general templates, and the sin of placing images of unveiled women in them falls upon the one who does so, not upon you. There is no harm in correcting errors unrelated to the images of unveiled women, such as font size and colors. Your work is permissible, and any resulting error is the action of someone else.