buttons in code blocks need a background

[Rich-Harris]

easy fix would be to add background: inherit on the button or .raised selector in the global styles, though it would probably look better if the space immediately around the button also obscured the text

[cowboycodr]

When experimenting with this I accidentally entered in the wrong background color. I kind of dig it. Thoughts?

[cowboycodr]

As opposed to something like this

[cowboycodr]

Just made a pull request. Went with the second option, keeping it consistent.

[cowboycodr]

Should be good to close with #601.