Default browsers’s monospace font for code blocks

In Appearance – Editor – Styles – Typography – Manage fonts I deactivated all fonts. I want my blog to be simple and don’t want to increase its size by using custom fonts. Now the site uses the default browser’s font and I’m happy with that.

Except that Code blocks are not using the browser’s default monospace font but instead are in the same font as the other text on the website. How can I change the font of the code block to be monospace, without activating any of the custom fonts of Twentytwentyfive?