Allow main-h1-bg, logo-margin-top configuration

[bobheadxi]

This PR adds a new item to _variables.scss to allow configuration of main-h1-bg, for example:

// ...
$main-bg: #F3F7F9 !default;
$main-h1-bg: #c97a7e !default; // this is some kind of red
$aside-notice-bg: #8fbcd4 !default;
// ...

now you get:

and for logo margins, if you change it slightly:

$logo-margin-top: 16px !default; // margin above logo
$logo-margin-bot: 0px !default; // margin below logo

you now have a much nicer spacing on the navbar:

[lord]

Hi Robert! This is a cool addition, and while I'm sure many users would like to change this color, it's also the kind of change they can make directly without using variables? I'd rather not avoid adding too many variables for every possible styling change, since the end result of that is a variable for every CSS rule that we have. If you think this is a particular pain point, perhaps it could be documented in the wiki?