Increase left side margin in posts

Something like this might do that, but possibly not for all viewports:

#content {
    padding-left: 20px;
}

I do not see much help being offered at your theme’s support forum, but you might find something there in a previous post:
https://ww.wp.xz.cn/support/theme/nirvana

edit: If you do not already have a Child Theme, a Custom CSS plugin might work here:
https://ww.wp.xz.cn/plugins/search.php?q=css

@js Daly

even if are you using the same theme, your issue is likely different, so please start your own topic.

http://codex.ww.wp.xz.cn/Forum_Welcome#Where_To_Post