refactor(cc-dark-theme): organize styles in several files

Until now we had a single file for the dark theme styles with ~2500
lines, with several sections which were marked with CSS comments.

This CL separates this file into several ones to improve its
maintainability, using the folder structure suggested by
https://sass-guidelin.es/#architecture.

Fixed: twpowertools:21
Change-Id: I0c1741f58b470085c0a499b32aa64feb43d8ad31
diff --git a/src/ccDarkTheme/layout/_index.scss b/src/ccDarkTheme/layout/_index.scss
new file mode 100644
index 0000000..c50e4e1
--- /dev/null
+++ b/src/ccDarkTheme/layout/_index.scss
@@ -0,0 +1,2 @@
+@forward 'header';
+@forward 'main';