I have encountered a bug as follows:
I created a Quote element and set the following as the citation:
(A healthy body in a healthy mind)
Which then caused a warning to show up in the frontend (and in the builder after a refresh):
Warning: Cornerstone CSS parser unable to identify value type: (A healthy body in a healthy mind) in /********/themes/pro/cornerstone/includes/classes/Tss/Environment.php on line 139
Removing the parentheses from the citation fixed the issue and removed the warning.