fix:put a background color to the left panel when the new room list is enabled (#30328)

Author: florianduros

res/css/structures/_LeftPanel.pcss

@@ -28,12 +28,6 @@ Please see LICENSE files in the repository root for full details.
     --collapsedWidth: 68px;
 }
 
-.mx_LeftPanel_newRoomList {
-    /* Thew new rooms list is not designed to be collapsed to just icons. */
-    /* 224 + 68(spaces bar) was deemed by design to be a good minimum for the left panel. */
-    --collapsedWidth: 224px;
-}
-
 .mx_LeftPanel_wrapper {
     display: flex;
     flex-direction: row;
@@ -246,3 +240,10 @@ Please see LICENSE files in the repository root for full details.
         }
     }
 }
+
+.mx_LeftPanel_newRoomList {
+    /* Thew new rooms list is not designed to be collapsed to just icons. */
+    /* 224 + 68(spaces bar) was deemed by design to be a good minimum for the left panel. */
+    --collapsedWidth: 224px;
+    background-color: var(--cpd-color-bg-canvas-default);
+}