threadListAvatars: honor transparency in avatars

This change honors transparency in avatars instead of forcing a white
background.

Fixed: twpowertools:70
Change-Id: I2f9fd170a2ecd8d81708fcf07327d494c4d1b778
diff --git a/src/static/css/thread_list_avatars.css b/src/static/css/thread_list_avatars.css
index 0289afb..7771bc0 100644
--- a/src/static/css/thread_list_avatars.css
+++ b/src/static/css/thread_list_avatars.css
@@ -17,9 +17,8 @@
 }
 
 .TWPT-avatars .TWPT-avatar {
-  background-color: white;
   background-position: center;
-  background-size: contain;
+  background-size: cover;
   background-repeat: no-repeat;
 }