How to remove dashed lines from GTK3 theme scroll areas

(For some reason, this video didn't get encoded to 1080p by Dailymotion's encoder.)

The GTK3 dashed lines in the scroll areas present in some themes has bothered me for quite a while, but I did some research into how to remove them. I found a way to remove them from all GTK3 themes, but I didn't want to do that in case it broke other themes, so I thought about it for a few minutes then realized that it should be possible to just take a copy of the theme and remove the dashed lines.

At first, I deleted the entire "undershoot" code block, but that left a blank area at the top and bottom of the scroll areas in the Cinnamon Settings application. During recording this video, I decided to actually try changing the background image of the undershoot on all sides to just be transparent instead, and it worked better, so I cut the first part out.

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(This video was edited with Adobe Premiere Pro CC 2015.4, Media Encoder CC 2015.4. Recorded with OBS. Thumbnail created with Microsoft Paint and Paint 3D.)

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