Posts with transparent bacground. Box size adjustment

Hey coders. I’ve a simple question that gives me headache for some time now. I’ve put transparent background on my blog post text area but it is way bigger that the text itself. I don’t know how to adjust the size of those background boxes to be the same size as the post. I would really appreciate if anyone could assist me. Here are the examples:
https://www.akvariumusodai.lt/apie-mus/
https://www.akvariumusodai.lt/akvariumo-prieziura/

Thanks in advance!