A WordPress site can have only one theme active, which in your case, is JupiterX. So, perhaps you’re referring to default post templates/styles or something, and not “post theme theme”.
In any case, your Jupiter X theme is a commercial product, so we have no way of installing it to check out the features and offer any assistance.
Please get in touch with the folks you purchased this commercial theme from. They built and sold the theme, and they should be in the best position to help you: https://themes.artbees.net/support/jupiterx/
Where do I find the CSS for it?
If you want to override any CSS style, first use your browser’s developer tool to identify the appropriate CSS selector, then add your custom CSS in the ADDITIONAL CSS box at APPEARANCE => CUSTOMIZE.
Good luck!
Hi George
Many thanks for your reply, I will write to Jupiter X.
Kind regards
Mads