If you have chosen a template that has a sidebar, there are a couple of ways to add borders to it. The easiest way to achieve this look would be to add a table to the block you are working in. Even if you just need a one cell table you can add it into the block and choose to add a border to it and change the colors. This will allow you to change the colors and thickness of the border when editing.
The other way to add borders is through the HTML Code of the block. If you are not familiar with code, then the above mentioned steps will most likely be easiest for you. If you are familiar with code, here is a way to add a border to a block:
When editing the block click the HTML button in the toolbar. You can then locate the style of the table (this will appear at the top of the Code View window). It will most likely have <table style="... to the style add this "border: 1px solid #000000;" without the quote marks. You can adjust the size of the border by changing the 1px to another number, and also the color by changing the hex value of #000000 to that of the color that you would like. Want to match this to a specific color, but don't know the hex value? Check out the section about selecting colors in this FAQ.
Hope this has helped, if you have any questions feel free to reply.
Thanks for posting! Are you looking to add borders around the entire email or just around individual blocks? We've got this FAQ to help with both of these topics!
Hope this helps!