You can modify the color of the Minimal and Standard Instruction borders by clicking the Theme tab.

You can change the Border and Back color for any Instruction or apply those changes to ALL Instructions.

The Border color and Toolbar Color color palette is the same palette used for text Forecolor and Text Highlight Color in the Formatting group of the Text Instruction ribbon. See the Setting content colors topic.

Remember, you can Preview these opening/closing Instructions while you are editing them to get a feel for how they will look in your content. Set the look, size, color, theme, etc.

The Resize button (4-headed arrow) on the Toolbar in preview mode does not exist in published content. See the difference below:

Preview Mode Toolbar

Published Content Toolbar