This method is exported so package users can generate their own rules
cf_field_to_css(cf_field, xview, css_fields, unlocked)
A cf_field object. This is like a rule, but with the computed colour values. It usually maps one-to-one to a CSS field.
A data frame with the columns to be printed and rows filtered
A list of matrices. The names of the list are CSS attributes and each matrix is of the size of xview and contains the respective CSS values.
A logical matrix of cells unlocked (that can still be modified by further rules).
A list with two elements: css_fields and unlocked (with updated values)