Dark theme: minor fixes

- Colored in a lighter blue the button in thread alerts (e.g. when a
  thread is a duplicate).

Change-Id: I285d01af8c3e0104fca7e005d541ea6eaeab2c23
diff --git a/src/static/css/ccdarktheme.css b/src/static/css/ccdarktheme.css
index f26c1ed..8a6b6ad 100644
--- a/src/static/css/ccdarktheme.css
+++ b/src/static/css/ccdarktheme.css
@@ -375,6 +375,10 @@
   fill: var(--TWPT-primary-text)!important;
 }
 
+ec-question .alert material-button {
+  color: var(--TWPT-link)!important;
+}
+
 ec-question .title {
   color: var(--TWPT-primary-text-alt)!important;
 }