Easiest way would be to open match in between highlights panel, find the container with that box, and add transparency="0.xx" and adjust that value until it looks good to you.
Agreed with what Tyburn has said.
If that doesn't work, follow the guide below https://support.sega.com/hc/en-gb/articles/4409224840081-How-to-remove-my-Preferences-and-or-Caches-Folder
easiest way would be to edit your settings file, edit the bg files
https://community.sigames.com/showthread.php/431357-Football-Manager-2015-Skinning-Guide-Part-3-Changing-the-Text-Settings
extract the file
To answer your question, few things.
A) What works for your resolution might not work for others with lower resolutions. The base skin requires a ton of empty space so the skin can scale from the lowest possible resolution.
B) Haven't looked at the competition screens yet and to be honest, i find the default one fine. Packing as much information into any space isn't what my skin attempts to do.
I think the easiest way to approach it would be to use the find in files function in Notepad++. Find a code used for the player faces, remove all traces, then when you spot another face, do the same until they're all gone