Mobile: Make banner full width

[rin]

David noted that we might want to make the banner full width on mobile:

But maybe, not? I don't know. If you like it like that, feel free to close this :).
@lulen11 @charliemckenzie

[lulen11]

I think we wanted to have the white border there team but good pickup! It was on purpose but I'll leave @charliemckenzie to have the final say.