Twenty Sixteen: Add gradient background options that fit the theme color scheme

[ianbelanger]

Now that we have the option for certain blocks to have gradient backgrounds, we need to add gradient background options to the theme that fit the themes color scheme.

[tomjdevisser]

Added theme support and four gradients

[tomjdevisser]

For this one there's also mostly black/white, so I went with the WordPress colors that would match anything existing

[karmatosed]

The plan for default themes currently is to focus on bugs and this coupled with the current approach for gradients using theme.json, means I would recommend this for closure.

I am going to put the keyword on so we have space for feedback if needed, but thank you everyone for collaboration so far. It's always good to consider after a while what might have changed with approaches and this is one thing we most likely would approach differently now if we did agree to do it.

[karmatosed]

I am going to close this as it was discussed in Slack, along with other tickets around gradients. I am closing them all together after having left them open for a few days with the close keyword for feedback. Thank you everyone for your collaboration.