removed background color from offline-warning.css and added wrapper class alert-info (#40237)

* fixed issue offline warning message is hard to read in dark mode

* Update client/src/components/OfflineWarning/OfflineWarning.js

Co-authored-by: Nicholas Carrigan (he/him) <nhcarrigan@gmail.com>

* Update client/src/components/OfflineWarning/OfflineWarning.js

Co-authored-by: Nicholas Carrigan (he/him) <nhcarrigan@gmail.com>

Co-authored-by: Nicholas Carrigan (he/him) <nhcarrigan@gmail.com>

client/src/components/OfflineWarning/OfflineWarning.js

@@ -25,7 +25,7 @@ function OfflineWarning({ isOnline, isSignedIn }) {
   }
 
   return showWarning ? (
-    <div className='offline-warning'>
+    <div className='offline-warning alert-info'>
       You appear to be offline, your progress may not be being saved.
     </div>
   ) : null;

client/src/components/OfflineWarning/offline-warning.css

@@ -3,5 +3,4 @@
   display: flex;
   justify-content: center;
   align-items: center;
-  background: lightblue;
 }