It looks like you may be on the Basic plan? If so, I’m afraid theme customisations are not available and you would need to upgrade to the Standard plan or higher to access that feature.
I’ve moved your posts from Change font sizes in your themes over to here to keep the discussion grouped together. 
It also seems you’ve asked this question/requested this previously. To change font size for everyone by default you can use the default text size admin setting, which has the added bonus of not needing access to theme customisations. 