Like any other incurable condition, the tendency for software suppliers to change interfaces for the sake of it and stop giving users the ability to revert to those they know makes using computers more of a chore or physical discomfort than it needs to be or should be.
In this case, it's Microsoft removing the "Light Gray" and "Medium Gray" themes from Office so you've got a choice of just three: "Colorful" header with overuse of white in Outlook, searing white, or very dark/inverted gray. It's really a continuing surprise that – especially in the US – more politicians, public faces of software companies, etc, haven't been attacked by members of the public (although at the time of writing a member of YouTube has just shot three people at one of their offices; interesting and insightful commentary on that: https://news.slashdot.org/comments.pl?sid=11946809&cid=56385393 summarising how YT have failed to understand that a decision to cut off income for people will nudge a few to try to kill those they hold responsible before themselves).
Anyway, eye strain, particularly for older users, isn't a joke. The newer interface is also bad for accessibility and consistent with the same arrogance that removed options from Windows 10.
Extra-helpfully, preview releases of Office 2016 did include other themes so there's a lot of information online that isn't accurate.
And thankfully that bloody stupid typing animation can be turned off, it just isn't a GUI setting.
The fundamental issue is software authors not respecting system themes, and restricting changes to system themes. Microsoft backtracked on removing coloured title bars in Win10, but Office has always had a problem with marketing disregarding user settings. This wouldn't work if Office is ignoring the Window colour setting, for instance:
https://superuser.com/questions/949920/window-color-and-appearance-removed-in-win10 HKEY_CURRENT_USER\Control Panel\Colors\Window
About the only solid hint I've come across is that Office respects some settings in high contrast "Ease of Access" themes, due to US disability legislation, so the thought occurs that marking a theme as high contrast but using more-or-less "normal" colours might be a way forward. Of course, if it was that simple, someone would have already done that and made the method popular.
It does have known downsides;
For now I've just adjusted the monitor Intel graphics settings on my work laptop so that white is no longer as bright, but I think it's something to look into when Windows 10 gets foisted on users. I got the OS set up with tolerable visual contrast on a personal laptop with a slightly lighter task bar than the stock colours allow for and dark grey title bars, and whilst Classic Start isn't really an option, a taskbar toolbar with a folder of shortcuts would substitute for the shitty excuse for a start menu that Windows now provides.