Rewrite the main course.scss file as well

Just as is done with the main LMS application.scss file, rewrite the
course.scss file with Mako to conditionally import a theme's
variables overrides. Add the course.scss file to the list of ignored
Git files so that it doesn't keep getting committed over and over
again.

This also requires us to add a hardcoded line in the assets Rakefile
for the moment, so that the course.scss.mako file gets properly
preprocessed. Once the preprocessing is done by a Django management
command, we won't have to do this anymore.

.gitignore

@@ -27,6 +27,7 @@ conf/locale/en/LC_MESSAGES/*.po
 !messages.po
 lms/static/sass/*.css
 lms/static/sass/application.scss
+lms/static/sass/course.scss
 cms/static/sass/*.css
 lms/lib/comment_client/python
 nosetests.xml

lms/static/sass/course.scss.mako

@@ -4,6 +4,20 @@
 @import 'base/font_face';
 @import 'base/mixins';
 @import 'base/variables';
+
+## THEMING
+## -------
+## Set up this file to import an edX theme library if the environment
+## indicates that a theme should be used. The assumption is that the
+## theme resides outside of this main edX repository, in a directory
+## called themes/<theme-name>/, with its base Sass file in
+## themes/<theme-name>/static/sass/_<theme-name>.scss. That one entry
+## point can be used to @import in as many other things as needed.
+% if env.get('THEME_NAME') is not None:
+  // import theme's Sass overrides
+  @import '${env.get('THEME_NAME')}';
+% endif
+
 @import 'base/base';
 @import 'base/extends';
 @import 'base/animations';