UBS Stewards have removed the My Custom CSS plugin from availability in favor of the newly introduced Aditional CSS functionality offered as part of WordPress Core.

From WordPress.org: https://codex.wordpress.org/CSS#Custom_CSS_in_WordPress

If you have been using the My Custom CSS Plugin to input your own CSS, these changes have been retained and moved into Customize > Additional CSS. From this interface, you can continue to input your cust CSS stylings with the added benefit of real-time preview capability offered in the Customize menu.