Steps
1) Click Templates
2) Click Edit button next to the template you want to edit
3) In edit mode scroll to the section you want to add a colored border to. You can add a border to sections not individual questions.
4) Click expand button
5) Click Border color, then slide the color bar. You can click around the color gradient until you find the color of choice.
6) If you want to apply the same color border to multiple related sections, copy the color id then paste it within the section you want to match by following steps 3-5.
7) Click one of the save buttons
Example
When applied, the border color will appear around the questions within a section