The Full Calendar jQuery plugin is themable using jQuery UI ThemeRoller.
Included Themes: flick, redmond
Customize and download your theme from jQuery UI ThemeRoller and zip the folder located within the css folder (this folder should contain two CSS files and an images folder). Upload the zip file into /_files/css/vendor/jqueryui using the Zip Archive tool.
Modify the block located at /_cascade/blocks/calendar head files. Update the CSS <link href="/_files/css/vendor/jqueryui/{THEME_NAME}/theme.min.css" rel="stylesheet" type="text/css"/> tag to point to the compressed (minimized) CSS file of the desired theme.
Note: It is recommended to publish the theme folder located at /_files/css/vendor/jqueryui before publishing the Full Calendar to ensure the styles and images are available.