Move CSS override to theme because RTD ignores layout.html

RTD bug: https://github.com/rtfd/readthedocs.org/issues/152

doc/_templates/layout.html

 {% extends "!layout.html" %}
 
-{# Custom CSS overrides #}
-{% set bootswatch_css_custom = ['_static/cppformat.css'] %}
-
 {%- block content %}
 {{ navBar() }}
 <div class="container">

doc/bootstrap/layout.html

 {% extends "basic/layout.html" %}
 
+{# Custom CSS overrides #}
+{% set bootswatch_css_custom = ['_static/cppformat.css'] %}
+
 {% set theme_css_files = [] %}
 {% if theme_bootstrap_version == "3" %}
   {% set bootstrap_version, bootstrap_additional_css, navbar_version = "3.2.0", "theme", "" %}