CTA buttons don't change color with custom masthead

[mattstratton]

Expected behavior

If a custom masthead image is used, the CTA buttons that overlay it are supposed to be not transparent.

Actual behavior

They are still transparent, because @mattstratton forgot to change that partial when he renamed the data field.

Reproduction Steps

View https://www.devopsdays.org/events/2017-chicago

[mattstratton]

Fixed in #457