[Dark Theme] Fix: highlight selected item in drawer

In order to stay consistent with the light theme, the background color
of the active item will be the same as the background color which
appears when you hover the items in the drawer.

Fixes: #14

Change-Id: I6fa4004e351d66697bbe57d3a5a107b4096abc5f
diff --git a/src/injections/ccdarktheme.css b/src/injections/ccdarktheme.css
index 9b80c7f..904339c 100644
--- a/src/injections/ccdarktheme.css
+++ b/src/injections/ccdarktheme.css
@@ -52,7 +52,7 @@
   background-color: var(--TWPT-drawer-background)!important;
 }
 
-material-list-item:hover, material-list-item:focus {
+material-list-item:hover, material-list-item:focus, material-list-item.item--active {
   background-color: var(--TWPT-active-background)!important;
 }