Found this thread through Google as I wanted to do was change the font size of the text in the sidebar.
Using the web inspector (Ctrl+Shift+I on Windows/Linux or Cmd+Opt+I on macOS) I was able to identify the classes to let me change it. Adding a css rule to a custom css snippet to target the following classes worked for me, and the font family can be changed as well. e.g.
.nav-file-title, .nav-folder-title {
font-size: 11px;
font-family: consolas;
}