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/_stepper.scss b/src/ccDarkTheme/components/_stepper.scss
new file mode 100644
index 0000000..5c035b1
--- /dev/null
+++ b/src/ccDarkTheme/components/_stepper.scss
@@ -0,0 +1,19 @@
+// The stepper is shown in the "new thread" and "edit message" views
+material-stepper {
+  border-top-color: var(--TWPT-card-border)!important;
+}
+
+material-stepper .stepper-step-name,
+    material-stepper .purpose-title {
+  color: var(--TWPT-primary-text)!important;
+}
+
+ec-ask-flow .display-name-label,
+    material-stepper .additional-details-label,
+    material-stepper .detail-label {
+  color: var(--TWPT-secondary-text)!important;
+}
+
+material-stepper .stepper-step[selectable=true]:focus {
+  background-color: rgba(60, 64, 67, .24)!important;
+}