Hi Tia,
Do not edit plugin files like the CSS file you noted. If you do that, and you update the plugin, your version would be overwritten by the update. That’s why you never mess with core wordpress or plugin code or files in general.
One place you can put CSS is in your child theme CSS file. But you may have inheritance issues depending on your theme and CSS code.
The easier place to put the example code is directly in the /customize page. Go to your page list, edit the “Customize”, go to the code view (Text tab) and paste it at the bottom of the page beneath any MyStyle shortcodes that may be in there. Do not remove the shortcodes. Here’s the example code again that you can paste directly into the page:
<style>
body.mystyle-customize {
min-width: 950px;
}
</style>