In addition to the standard setup options, you may also use Cascading Style Sheets (CSS) language to change the look of the portal border. Here is an example:
Portal Page BEFORE CSS:
Go to Admin > Properties. Open your property.
Go to Resident Portal Settings.
Add any desired CSS language. Here is an example of changing the border color to blue:
Here is the Portal after these instructions:
Here is a helpful link to learn CSS: http://www.w3schools.com/css/