Dark theme: load Roboto Bold

In the dark theme, Roboto Bold (700) was not loaded, and this caused
bold text to not stand out. By adding the bold font, now bold text is
shown in real bold, which stands out a little bit more.

Change-Id: I6b2a6ccfb308cc9b6fd336a8cdf6146ca68e220d
diff --git a/src/injections/ccdarktheme.css b/src/injections/ccdarktheme.css
index 6857eb8..5524f8a 100644
--- a/src/injections/ccdarktheme.css
+++ b/src/injections/ccdarktheme.css
@@ -1,3 +1,5 @@
+@import url('https://fonts.googleapis.com/css?family=Roboto:700');
+
 :root {
   --TWPT-primary-text: #e8eaed;
   --TWPT-primary-text-alt: var(--TWPT-primary-text);