Using the following two threads, I was able to change font size for the folders pane and the outline pane. The CSS in the first thread is specific to the folders pane. The second thread explains how to open the Web Inspector to find the CSS class for the specific pane that you want to change font size for.
If you’re not familiar with the Web Inspector, once it’s open, choose the cursor icon (far left) and then hover over the element that you want the CSS class for. The class will be highlighted in the Inspector. Use that class in your CSS file.