[Theme: Mantra] Oembed YouTube widths

I’ve been trying to set it up so that when I paste a YouTube link in a post, it shows up at a certain size so that it will fill the content area. Right now it defaults to 500 pixels wide on desktops, laptops, and tablets.

I’ve had no luck. I’ve tried changing the functions.php. I’ve tried plugins. I’ve tried child themes. Nothing changes the default size. I’ve looked through every setting, the source code, and still nothing.

Can anyone tell me what this theme needs so that I can make my oembeds the width I want? I’m not the main poster on the site, or I would just use the YouTube embed codes at this point.