Hi Kyle -
You can do this stuff with CSS. What I find to be easiest is to add the property to the landing page, approve it, and then open the page in google Chrome. Right click on the rectangle and say "Inspect Element".
Once I did that I saw that the rectangle was named with class "boxSpan", but I bet this is a generic class that applies to all rectangles on marketo lps, not JUST this specific box. If you want to only target a single box on the page then you'll need to be more specific at targeting the naming. Google Chrome can help you figure that out.
Then I just added a custom HTML block, used < style type="text/css" > and the class boxSpan...
.boxSpan {border-radius:12px;box-shadow:12px 12px 12px #069;}
Which makes the box now look like this.
Changing the form field text is harder. I just gave it a good 5 minute try but the css is being overwritten by the LP styles css file. //shrug// it would take more than 5 minutes to figure out.
Remember, Google Chrome is your friend.