I'm somewhat reluctant to get involved with this type of thread, I do understand how people can be sensitive regarding theme modification.
The white line is indeed by design and is included in the window frame caption images, both the Per Pixel and UIS2 frames. There is nothing to "fix" in that regard but you may certainly modify the images for your own personal use.
Restrictions: Modifying the images and/or themes is for your own use and amusement only. Only the original artist has the right to distribute his/her images without permission from that artist, regardless of whether the images are in an original or modified state. That means a person is not permitted to use another person's original or modified images without permission, either individually or in combination with other images, within anything that he/she chooses to share. That also means no one other than the original artist is permitted to modify an image for a person and distribute it to him/her.
Also, if you modify a skin, it would not be reasonable to expect the original artist (or anyone else) to provide specific help on that skin afterward. You may get some help via the public forum, but that is not necessarily to be "expected".
That is my understanding of the rules, not a legal quote. Hopefully that has it covered. If not, someone can feel free to correct me or expand.
If you want to modify the images for your personal use you would use SkinStudio and an image editor. There is a SkinStudio tutorial here:
https://forums.wincustomize.com/447931/page/1/#3499153
Happy tweaking.
