Make GlBanner follow design system again

!1280 (merged) revealed that more components than we are expected do not follow design system. GlBanner is one of such components, we need to update CSS to make it compatible once again

Figma specs: https://www.figma.com/file/qEddyqCrI7kPSBjGmwkZzQ/Component-library?node-id=19944%3A6