One of the options that Steam has is to be able to change the interface design through skins.
The first thing is to find one that we like (below I will put some pages where you can search) and download the compressed file.
Once downloaded we open it and unzip the folder in the skins folder (in my case in ~ / Steam / skins) and in the Steam client we are going to View -> Parameters (or we click with the right mouse button on the icon in the tray system and select the same option) and in the Interface tab we open the drop-down where it says Select the appearance of Steam and select the skin.
Once selected, click on OK and it will tell us that we have to restart the client. We click on Restart and we only need to restart to see how it looks now.
I hope you liked this mini tutorial ^^
Thread on the Steam forums with various skins
Page with skins for both Steam and some of the games
Minimal Steam UI V3 (the theme I use with several colors to choose from)
If you know any interesting skin or a page where you can download more you can post it in the comments