Plugin Tag፥ borders
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Bordered Blocks – Add subtle borders or outlines and labels to Blocks in the Editor
Bordered Blocks adds subtle borders to all blocks in the WordPress Post/Page editor to give you a much better overview of the layout of your page.
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Admin Classic Borders
Starting with the MP6 plugin, and by default in WordPress 3.8, the admin backend has been flattened. This plugin restores classic (3D) borders.