I came across this post when I was searching for a way to customize google call on one of the websites. Finally came out with my own solution. So if that’s what you are looking for ncase – try it out.
All you need to do is go to plugins/stout-google-calendar/gcalendar-wrapper.php
In line 256 change
$headEndTag = '</head>';
$calCustomized = preg_replace("/".preg_quote($headEndTag,'/')."/i", $calCustomStyle.$headEndTag, $calCustomized);
to
$headEndTag = '</head>';
$addCustomStylesheet = '<link rel="stylesheet" type="text/css" href="https://www.yourdomain.com/wp-content/themes/blah/cal_style.css" />';
$calCustomized = preg_replace("/".preg_quote($headEndTag,'/')."/i", $calCustomStyle.$addCustomStylesheet.$headEndTag, $calCustomized);
where https://www.yourdomain.com/wp-content/themes/blah/cal_style.css is path to css file responsible for css for cal