Yep, each skill card and each hero card of a particular rarity has a fixed strength value. Afaik, skills and heroes have a different strength value for the same rarity.
And of course, only the cards currently assigned to your deck contribute to your deck strength.
I’ve been killing pirates on a regular basis when visitng friends, and the victory message states I’ve won x green or y yellow crystals but I never see them. Reading through the forum, I understand these are to improve the deck but I see no evidence of it happening.