This change applies the default button styles to all button blocks in Twenty Sixteen, instead of only targeting those in `entry-content`. This means that any buttons added outside of `.entry-content`, such as in a widget area, will be styled the same as any other button blocks. This change also makes the theme custom colors work outside of `.entry-content`.
Follow-up to [44306].
Props mikachan, costdev, audrasjb.
Fixes#55167.
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