That’s the page for your custom CSS, not the page for your theme’s CSS, which is where this is coming from. You’ll have to edit your theme’s templates and CSS.
On the Deisgn page there is nothing which matches the screenshot you have provided. Attached is a screenshot of what I am seeing when I go to Design. There is no option to “Edit Themes”
Custom themes aren’t included in the $1/month Micro.one subscription, which is why things might look a little different for you. To access custom themes and other features, you’ll need to upgrade to the $5/month Micro.blog subscription.