move z-index setting to zero instant container, and add ability for zindex to be set on container

packages/instant/src/components/zero_ex_instant_container.tsx

@@ -5,13 +5,20 @@ import { ColorOption } from '../style/theme';
 import { BuyButton } from './buy_button';
 import { InstantHeading } from './instant_heading';
 import { OrderDetails } from './order_details';
+import { SlidingError } from './sliding_error';
 import { Container, Flex } from './ui';
 
 export interface ZeroExInstantContainerProps {}
 
 export const ZeroExInstantContainer: React.StatelessComponent<ZeroExInstantContainerProps> = props => (
     <Container width="350px">
-        <Container backgroundColor={ColorOption.white} borderRadius="3px" hasBoxShadow={true}>
+        <Container
+            zIndex={2}
+            position="relative"
+            backgroundColor={ColorOption.white}
+            borderRadius="3px"
+            hasBoxShadow={true}
+        >
             <Flex direction="column" justify="flex-start">
                 <InstantHeading />
                 <OrderDetails />