Docs: box on tabbed content

[rosibaj]

It's difficult to see what content goes with the tabs.

• Please add a rounded corner border around the content
• Also use the darker green for the active tab text color to make more accessible: #15846C

• please test- in this example, the page jumps around when I click the tabs, losing my place on the page. The other tabs seem ok. Can you tell why this is happening? Seems to be when the tab content is short
  

Originally posted by @kcullion in #398 (comment)