The scrollbars have their own theme and do not adopt e.g. the default Breeze theme. For example, they are not rounded on the ends and on hover-over with cursor, they do not change to blue (default Breeze color scheme), but rather change to a darker gray. This is true of both the composer and message view windows (and message pane).
KDE Neon Developer Edition (git stable)
Sorry, the composer window is fine, it is only for the view window. I've attached a screen shot.
Created attachment 102155 [details]
Difference between composer and viewer scrollbars
That's a QtWebEngine problem. Please report this issue directly to Qt developers via https://bugreports.qt.io/
Cheers. This has already been reported: https://bugreports.qt.io/browse/QTBUG-51186
*** Bug 370555 has been marked as a duplicate of this bug. ***