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/components/_duplicate-thread.scss b/src/ccDarkTheme/components/_duplicate-thread.scss
new file mode 100644
index 0000000..cf0858f
--- /dev/null
+++ b/src/ccDarkTheme/components/_duplicate-thread.scss
@@ -0,0 +1,17 @@
+.search-results ec-thread-option material-expansionpanel .panel {
+  background-color: var(--TWPT-primary-background)!important;
+}
+
+.search-results ec-thread-option material-expansionpanel.selected .panel,
+    .search-results ec-thread-option material-expansionpanel .panel > .main-header > .header.closed:hover,
+    .search-results ec-thread-option material-expansionpanel .panel > .main-header > .header.closed:focus {
+  background-color: #17191c!important;
+}
+
+.search-results .results-title {
+  color: var(--TWPT-primary-text-alt)!important;
+}
+
+.search-results ec-thread-option material-expansionpanel .panel > .main > .main-content .snippet {
+  color: var(--TWPT-secondary-text)!important;
+}