Use a transparent background for the DrawerTab component. (#18)

This will hopefully resolve an issue with the `light` theme which was
resulting in a dark background making it impossible to read the labels on
the components (e.g. "HTTP Headers" and "Query variables").

packages/graphql-playground-react/src/components/Playground/GraphQLEditor.tsx

@@ -783,7 +783,7 @@ interface DrawerTabProps {
 const DrawerTab = styled<DrawerTabProps, 'button'>('button')`
   padding: 0;
   margin-right: 10px;
-  background: #0b1924;
+  background: transparent;
 
   text-transform: uppercase;
   font-weight: 600;