Dark theme: fix regression in thread list checkbox color

CL:1760 introduced a regression: while it fixed the color of checkboxes
in the drawer, it also unintentionally changed the color of checkboxes
in thread lists. This CL fixes this.

Change-Id: I74cc911c94c0823fe71a860b586c95357ccbff76
diff --git a/src/static/css/ccdarktheme.css b/src/static/css/ccdarktheme.css
index 3b580e7..a88fd7b 100644
--- a/src/static/css/ccdarktheme.css
+++ b/src/static/css/ccdarktheme.css
@@ -830,13 +830,13 @@
 }
 
 /* Checkbox input (except for forum checkboxes in the drawer) */
-material-checkbox material-icon:not(ec-forum-drawer-item material-icon),
-    material-checkbox .content:not(ec-forum-drawer-item material-icon) {
+material-checkbox material-icon:not(ec-forum-drawer-item material-icon, ec-thread-list material-icon),
+    material-checkbox .content:not(ec-forum-drawer-item material-icon, ec-thread-list material-icon) {
   color: var(--TWPT-secondary-text)!important;
 }
 
 /* .gm-icons is added by the SMEI_GOOGLE_MATERIAL_ICONS experiment. */
-material-checkbox material-icon.filled:not(ec-forum-drawer-item material-icon) {
+material-checkbox material-icon.filled:not(ec-forum-drawer-item material-icon, ec-thread-list material-icon) {
   color: var(--TWPT-blue-A100)!important;
 }