KickstartCassiopeia has the option to use rounded corners on lots of elements all at once.
This design choice can benefit your website. Known as contour bias (see this page from the book Universal Principles of Design), rounded corners make objects appear less harsh and more friendly. The book cites the seminal work on contour bias as being this article Humans Prefer Curved Visual Objects by Moshe Bar and Maital Net.
However, note that rounded corners are not necessarily the right answer - the book also says:
Objects with pointed features, elicited stronger activations in the region of the brain related to associative processing, meaning that although angular objects were less liked they elicited a deeper level of processing than did the contoured objects - they were in effect, more interesting and more thought-provoking to look at
So perhaps it is a question of whether you wish content to appear more friendly or more noticeable. As the book continues:
Angular objects are more effective at attracting attention and engaging thought; contoured objects are more effective at making a positive emotional and aesthetic impression.
To set this option, go to your module, and under the tab 'General' you can set the 'Content border radius' from 0 (no border radius) to 2 (max radius)
CSS elements that are currently affected by this setting are:
More on this topic can be found here.