Standalone adjectives

Noeline, 

the rule applies only to stand-alone expressions of the form ' c'est (+/- adverb) adjective ' -  being " general " expressions, without a noun. 

In this structure ' ce sont ', the plural form of "c'est",  is not used, and the singular masculine form of the adjective  will always apply following ' c'est '. 

Of course, if using ' ce sont ', or indeed informally using ' c'est', followed by plural noun-adjective combinations, the adjective will agree in gender and plural form with the noun.  ( Adverbs are invariable regardless ).

See the attached lesson and links to Laura Lawless site for a discussion of rules and realities of using ' c'est ' or ' ce sont '.

C'est, ce sont = this is, these are (French Demonstrative Pronouns) 

https://www.lawlessfrench.com/grammar/cest-vs-il-est/ 

 https://www.lawlessfrench.com/expressions/cest/