Outlook 2016 and the lack of colour options.
I spend a lot of time behind MS-Outlook and find a straw coloured background (like in PPrune) is gentler on my eyes. However, in their lack of wisdom, and I suspect it's more to do with marketing than usability research, Microsoft now don't allow you to choose your own background colours for the folder navigation, mail lists and calendar preview. You can change the mail message background but that then makes the whole layout look weird.
Apparently, many MS-Outlook 2016 users are already complaining to Microsoft about this, and their KB only refers to the 3 colour schemes that are built-in to MS-Outlook.
This is really poorly thought out by Microsoft.