This problem appeared some time ago so it should not be news, but as it persists across numerous versions and I'm annoyed enough to register an account today, time to finally file a bug on it but can't place the version it appeared. The scrollbar in the message list is drawn normally (using the correct theme). The scrollbar for the the message view, whether it be below the message list in the same window or opened in a separate window, is drawn "unstyled". By unstyled I mean it's not my theme and it's not the default theme (which some things insist on against user preferences). Instead what should be a scrollbar is a plain light grey rectangle on a lighter grey background. There is almost no contrast so seeing the scrollbar is an exercise in frustration. How hard is it to use the same scrollbar rendering through the application? Apparently too hard for KDE devs...
Your last paragaraph isn't helpful. KMail's message viewer does not follow the system theme because it uses QWebEngine to display the message, in order to use the supported Qt component (QtWebkit is considered to be obsolete) and to fully support HTML mail. QWebEngine (which is really Chromium internally) draws its own scroll bars and does not use the desktop theme. The same can be seen with Akregator's article view, or Konqueror with the WebEnginePart. Leaving this bug open, in case a solution can eventually be implemented.
Your first paragraph isn't helpful... In all seriousness though, this defective scrollbar rendering is like a plague slowly moving through KDE apps. Making a consistent GUI isn't all that hard, especially with Qt at your disposal, so it looks rather much like incompetence. Hearing it is caused by QtWebengine only worsens the situation as it becomes obvious that Google is gaining access to an ever greater amount of data without user consent. Nevermind the other scrollbar inconsistencies added in various other places in KDE without use of QtWebengine. How about an option to display email with QtWebengine (obviously flawed and totally untrustworthy due to Google spyware embedded), QtWebkit (still supported), or a plain text viewer (safe, sane, and what should be default)?
Falkon browser developers found a way to use native scrollbars together with QtWebEngine, see bug 394813 comment #1. Maybe we need a KWebEngine wrapper to share code needed to use QWebEngine in KDE applications. Regarding being able to switch between different viewing engines: If you find the time to contribute and maintain a plain text viewer backend for KDEPIM, feel free to explain your proposal to the kdepim-devel mailing list. QtWebKit is not supported.
See also bug 376919 (reporting same problem also with Akregator and Konqueror). As mentioned there, Falkon (formerly Qupzilla) does indeed have an option for using native scroll bars, but there appear to be performance problems. See https://github.com/QupZilla/qupzilla/issues/2246
Any news on this? I didn't realize it's such an old standing known bug. It is pretty annoying visually.