Added env option for setting dark mode default

Also allowed config-centralised default user settings for this change
and bought existing user-level view options into that default settings
system to be cleaner in code usage.

For #2081

resources/sass/_html.scss

@@ -25,7 +25,6 @@ body {
   line-height: 1.6;
   @include lightDark(color, #444, #AAA);
   -webkit-font-smoothing: antialiased;
-  background-color: #F2F2F2;
   height: 100%;
   display: flex;
   flex-direction: column;