A semi-large-scale improvement in how the theme is structured.

I noticed that whitespace doesn't scale as nicely on smaller displays (phones/tablets) as the default octopress theme does. In whitespace upon width constraint the text gets pushed up to the edges of the display - for more infos on how the default octopress theme (and after this commit, also whitespace) handle this more elegantly see the *"Maintain Good Phone and Tablet Scalability"* section [in this post](http://tutorials.chymera.eu/blog/2013/12/30/octopress-theme/). 

Trying to implement all of this I ended up making some more changes which take content out of ```/sass/custom/_styles.scss``` and put it into more appropriate locations. It is best not to use ```/sass/custom/_styles.scss``` at all, as it overrides all other specs and thus makes the theme a bit less responsive.

Summary of changes:

* Now setting colors (```$nav-bg``` and colors for navigation borders) in the appropriate file ( ```sass/base/_themes.scss``` ).

* Setting (curtailed) navigation bar specifications in the appropriate file ( ```sass/partials/_navigation.scss``` ).

* Delleted color specifications in ```/sass/custom/_styles.scss``` which override end-user color specifications in ```sass/custom/_colors.scss```; also removed navigation specifications from here as they were placed in a better (more transparent and conducive to site responsiveness) location -  ```sass/partials/_navigation.scss``` (see above).

* Now defining margin instead of padding in ```/sass/base/_layout.scss``` so that the distance from the navigation and article content scales nicely with smaller displays (phones, tablets).


I tested it and it works nicely, but **please test it yourself as well, as it is a rather big commit!**