KickstartCassiopeia has the option to create rounded buttons on your website.
In order to create a box shadow on certain elements you can use CSS. This tutorial explains how you can create a neat shadow for your boxes.
By default users cannot register unless you enable them to. This is a Joomla setting.
Setting up your blog has way more options in Joomla4, and with the new Cassiopeia template. You can even select blogs based on tags.
Your site has some commonly used modules already preconfigured. These include: