fix transparency bug for sidebar in mobile (#6764)

dev/bots/docs.sh

@@ -2,8 +2,7 @@
 set -e
 
 # Install dartdoc.
-# We peg to this version to work around a bug affecting Flutter in 0.9.7+2.
-pub global activate dartdoc 0.9.7+1
+pub global activate dartdoc 0.9.7+6
 
 # Generate flutter docs into dev/docs/doc/api/.
 (cd dev/tools; pub get)

dev/docs/styles.html

@@ -50,6 +50,16 @@
     font-size: inherit;
   }
 
+  .sidebar-offcanvas-left {
+    background-color: #fdfdfd;
+  }
+
+  @media screen and (max-width: 768px) {
+    .sidebar-offcanvas-left.active {
+      padding: 10px;
+    }
+  }
+
   .sidebar-offcanvas-left ol {
     padding: 0 16px 16px 0;
   }
@@ -58,10 +68,6 @@
     display: none;
   }
 
-  .sidebar-offcanvas-left {
-    background-color: inherit;
-  }
-
   .sidebar-offcanvas-left h5 {
     border-bottom: none;
     padding: 0 0 16px 0;