Control over thumbnail appearence

[carlca]

Hugo v0.31.1
Icarus (just downloaded/installed)
Firefox Quantum

Hello, I am trying to use CSS to change the background color of the "Recents" links, the grey blocks in the attached image:

I can control the size of the blocks by altering my custom CSS as shown:

#recent-post li .item-thumbnail .thumbnail {
  width: 40px;     // Changed
  height: 40px;   // Changed
  display: block;
  position: relative;
  overflow: hidden;
  background-color: #c0d0ff;   // Failed
}

But my attempt to change the background-color has failed miserably! I'm not very skilled with CSS so go easy on me!