hi there
i am running KDE neon 5.15 User Edition, KMail 5.10.2
the message preview pane's scrollbar style is gray and rectangular (e.g., see bottom right of image at https://kontact.kde.org/components/kmail.html)
whereas, the message list's, folder list's and configuration settings' scrollbars match the Breeze widget style
how might I get the message preview pane's scrollbar style to match the others' scrollbar style, as displayed similarly in an image at https://kde.org/applications/internet/kmail/?
in other words, how might I get the message preview pane's scrollbar style to match the Breeze widget style?

That is a good question. I'm using it right now with a custom theme and color scheme and it is still using a plain rectangular light-gray scroll bar. I suspect it may have to do with Kmail's use of an html renderer; though I'm sure other Qt-based browsers still use the theme's scrollbars.

I'd suggest filing a bug at bugs.kde.org.

i've settled on the possibility that the qtwebengine may not have a proper api to handle embedded scrollbars
I submitted bug report 391327 (see https://bugs.kde.org/show_bug.cgi?id=391327) for a separate issue, but I suspect that the cause of that issue may be related to the cause of this issue. i'm not certain, but it seems reasonable. why i also think it's related is because even firefox shows embedded scrollbars with the light-grey color.