Well the issue is this line is missing from your CSS
border: 0 none;
As for why, I'm not 100% sure.
Can you try running
./nodebb restart immediately followed by
./nodebb log and seeing if it logs any errors minifying the CSS.
Can you also check what version of the plugin and what version of NodeBB you are running. Go into your node_modules folder, followed by the nodebb-plugin-youtube-lite/static/ folder and check you have a file called style.less. If this isn't being minified with your stylesheets on NodeBB load, then you will see this issue.
Also thanks for using my plugin.