Newbie Problem - Font size too small in Sidebar and Tag Pane

Sorry, I’m still struggling. It has taken me a couple of days to find out why my Vault had no Obsidian or root folder. Eventually I discovered that this folder is hidden on my MacBook Pro and that it can be revealed by CMD SHIFT Period so that problem is solved.
I am still confused about exactly what you are asking me to do.
I think you suggest I create a new folder in the snippets folder. If this correct, what name do I give the folder? Presumably the .css extension does NOT go on the file name because I tried that and got a warning message. So where does the .css extension go? finally what do I do with the block of code you have given me? Copy it as is into the new folder?
I know you will be grinding your teeth at me by now but please clarify all the necessary steps to get me going.
Again, many thanks your help. I cant wait to see my Sidebar more easily!
All best wishes,
David