To me the darker separators clearly say they are just for visually dividing the different elements. The lighter ones look like they can be interacted with (maybe they can?) but also are more easily confused with scrollbars, especially when right next to actual ones.
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@Hadriscus@jlai.lu - what do you think?
Btw I've deepened the contrast as a whole based on your feedback - both in this and the PrintroomExpertSuperflat theme - and am happier with them.
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