Hello!
I believe the problem is that your custom CSS rules are loaded before the CSS rules from the theme, so rules from the theme get a higher "specificity" and override your rules.
What you can do is add extra tags or classes to the rules to increase their specificity. For example, use "html body" instead of just "body".
You can read more at
https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity