You are in over your head, but no worries, that’s exactly how people learn this stuff … long, often-frustrating nights. One problem is that you are trying to do a number of (related) things at once. Too many moving parts for you to handle at the moment. The lights will go on over time, but it takes time. I had, and still have, plenty of WTF moments! 🙂 Inspectors were invented for a reason. Layout with css would be very, very hard without them.
Assuming your theme is called simplify, you should be using
/public_html/wp-content/themes/simplify/single-player.php
You are correct about the .css file.
Does what override what? The page templates and the .css work together, they don’t really ‘override’ each other. Stylesheets do ‘override’ each other by ‘cascading’, but that’s a bit more complicated. All the necessary tools are built into the plugin, but once you stray from the Settings screen, you really need to know what you’re doing with css selectors. (Read about them.)
You are correct about the inspector and going back and forth with the stylesheet. Besides modifying rules, there is a + button in Chrome’s inspector that let’s you create and apply a whole new rule. I often to that, get it just right, then cut and paste into the stylesheet.
You change the height in the rule that I showed you in the screen shots. height: 162px; was remarked out.
-Mark