User blog comment:Rappy 4187/Technical Update: September 21, 2016/@comment-24258073-20160926050547/@comment-1251315-20160928093514

@Noreplyz: It's true you can do that, but it's not a practical solution and only applies to the user who installs the extension (the user might not even choose to do that), so including site-wide JS/CSS into /d for users would be very helpful for improving the odds of people accepting /d (since users would be able to customise parts that are lacking to add something that could become core later).