If you want to have custom styles on your Barclays payment page, you need to create your dynamic template page.
Here is example of dynamic template payment page:
You can change this page as you want, only part which must be there is
as this part is then replaced by Barclaycard code.
Then in Barclaycard admin you need to enter path to this custom template page:
Path to this page you need to insert also to your Magento admin settings. We are using Barclays Payment Integration module from Juno Media:
After this you should see your custom payment page after you click on Place Order and you are redirected to Barclay.
You can find more information about Barclay payment gateway in attached pdf document.