With macOS 26 NSRulerViews are showing a very different layout, not only in contrast to prior macOS versions, but also inconsistent between horizontal and vertical rulers. I observed this issue with one of my apps, but was able to replicate it with the attached minimal sample app, using a scrollable text view with rulers, whose scrollview's background was set to orange. For your convenience I attached a screenshots with the result.

• The horizontal ruler view is showing with a solid color. I assume the color is selected based on the first line of the inner view, because with a transparent content view the scrollview's background color is used, otherwise the content view's background color.
  
• The vertical ruler view is using a much lighter or transparent version of the content view's background, what I would consider the intended (and expected) behavior.
  

RulerTest - Window.png