Custom Background Behind Logo

Hello WordPress Support!

I am brand new to the blogging scene and am working on a site for my wife. I have downloaded and installed the theme named Amadeus and this theme has been extremely easy to use and has been awesome. However there is 1 thing (so far) that I have not been able to change to my liking and that is the logo. I would like for the logo either to have a custom background behind it instead of a solid color or I would like for my logo to fit across the entire width of the screen. I say either or because if the logo fits across the width of the screen, then I can just implement a background behind my logo image. The best solution to this problem would be for there to be a custom background. I just don’t know if this is possible.

I looked through the support forums and was not able to find a solution to this issue. One post came up with a solution to change the logo width, using the code:

.themonic-logo img {
	width: 100%;
}

However this did not work for me.

I do have some limited CSS knowledge and if you can give me a piece of code that needs to be implemented I can certainly do that but I would need to know which file to place it in. Also, my website is not up and running yet because I want to have the site presentable before I go live.

Thank you so much for all of your help on this issue! I hope we can get it figured out.