Add extra CSS styles to a registered stylesheet.
Styles will only be added if the stylesheet is already in the queue. Accepts a string $data containing the CSS. If two or more CSS code blocks are added to the same stylesheet $handle, they will be printed in the order they were added, i.e. the latter added styles can redeclare the previous.
(string) (Required) Name of the stylesheet to add the extra styles to.
(string) (Required) String containing the CSS styles to be added.
(bool) True on success, false on failure.