[Dark Theme] Improve thread list backgrounds

Selected threads now have a background color which can be better
distinguished from read threads.

Also, a bug has been fixed which consisted in the fact that threads
which had been read and checked at the same time had the background for
read threads instead of the background for checked threads.

Fixes: #15

Change-Id: Ibb577553e8a77e1b131e20afd637a041f69880ba
diff --git a/src/content_scripts/console_inject.js b/src/content_scripts/console_inject.js
index df20e79..d804459 100644
--- a/src/content_scripts/console_inject.js
+++ b/src/content_scripts/console_inject.js
@@ -132,7 +132,7 @@
 
   if (options.increasecontrast) {
     injectStyles(
-        '.thread-summary.read{background: var(--TWPT-thread-read-background, #ecedee)!important;}');
+        '.thread-summary.read:not(.checked){background: var(--TWPT-thread-read-background, #ecedee)!important;}');
   }
 
   if (options.stickysidebarheaders) {
diff --git a/src/injections/ccdarktheme.css b/src/injections/ccdarktheme.css
index 904339c..d481101 100644
--- a/src/injections/ccdarktheme.css
+++ b/src/injections/ccdarktheme.css
@@ -149,7 +149,7 @@
   border-bottom-color: var(--TWPT-subtle-border)!important;
 }
 
-ec-thread-summary material-expansionpanel.read {
+ec-thread-summary material-expansionpanel.read:not(.checked) {
   background-color: var(--TWPT-thread-read-background)!important;
 }
 
@@ -162,7 +162,7 @@
 }
 
 ec-thread-summary material-expansionpanel.checked {
-  background-color: #17191c!important;
+  background-color: #2c4b77!important;
 }
 
 ec-thread-summary material-expansionpanel .title {