fix(calendar): update part selectors to support multiple values

Author: desig9stein

src/components/calendar/themes/calendar.base.scss

@@ -139,17 +139,16 @@
     padding: rem(16px);
 }
 
-[part='content'] {
+[part~='content'] {
     display: flex;
     flex-grow: 1;
     border-radius: inherit;
     border-top-left-radius: 0;
     border-top-right-radius: 0;
 }
 
-[part='content-vertical'] {
+[part~='content-vertical'] {
     flex-direction: column;
-
 }
 
 [part='header-date'],