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/popups/_popup.scss b/src/ccDarkTheme/components/popups/_popup.scss
new file mode 100644
index 0000000..11f01e7
--- /dev/null
+++ b/src/ccDarkTheme/components/popups/_popup.scss
@@ -0,0 +1,8 @@
+// Generic popup (for notification bell, account selector, etc.)
+.popup-wrapper {
+  background-color: var(--TWPT-drawer-background)!important;
+
+  .header-text {
+    color: var(--TWPT-primary-text)!important;
+  }
+}