First, search the help docs and this forum. Maybe your question has been answered! The debugging steps can help, too. Still stuck? Delete this line and proceed.
What I’m trying to do
See in the options of settings, the appearance, but it only has changed the central page and not the side bar.
Things I have tried
Have you tried adjusting the zoom level up and and font size down (if the text font is too big after the zoom adjustment)? If that doesn’t work for you, there are CSS options to increase the sidebar font size.